Hello to all of you!
Hopefully you’ll excuse a newbie to this forum with a beginner-question 
Are there any social “better not” actions in Sigil? The question goes into the direction of this example: “spitting chewing gum on the pavement in Japan will cost you 200 bucks”
Thanks for helping to avoid any pitfalls! 
I think that in such planar metropolis like Sigil, would be extremly rude and unpolite to ask questions like: "what is your race", or even worse: openly calling someone "monster", or stare preplexed at tentacled thing that is trying to enjoy its lunch at restoraunt while evryone is staring, etc...
"Monsters" are rightfull citezens of Sigil too, and Sigil citizens are some of the worst snobs in multiverse, so they can be easily ofended by primes ignorance.
Never cast know alignment, or if you do, keep it to yourself, berk. Asking a being its alignment is also taboo.
As a house rule, nobody in my Sigil ever mentions the Faction War directly.
And for your own sake, don't call a Baatezu a "demon", a Tanar'ri a "devil", or a Gereleth a "daemon", or the'll show you why the Blood War starts with "blood".
Don't call a Tanar'ri a demon and a Baatezu a devil either. Even though fellow primes will no longer think you ignorant, it won't make much difference to a cager.
I think some lead sheeting is in order.
Do not mention A'kin anywhere within earshot of Shemmy.
Drake
Githyanki.
Githzerai.
Illithids.
If you're not one of these 3 races, think twice before having dealings with any of 'em (but especially the flayers.)
If 2 or more of these races are in the same place at the same time, don't be anywhere near 'em, berk.
Or, if you really must say 'demon' or 'devil' make absolutely certain you call don't call a Baatezu a 'demon' or a Tanar'ri a 'devil.' That's not smart.
Ahh, some wise advise! Thanks to all of you!
I'll try my best to avoid getting hit within the first minutes of my whistle-stop in Sigil ...
Also, taboo themes are: openly talking about her serenity, Lady of Pain. She dont mind gossip but some citezens will shut you up... permanently. It is just bad mojo.
Even worse: talking about Aoskar (dead portal god) and you are ivinting one way trip to dead book, you wont be flayed by Lady, citizens will finish you off long before she apears.
Also avoid talking to Dabuses unless you have to, they can be helpful but some people will asumme you are cousing trouble and will probably call Harmonium before more trouble brews... (they just dont want to be involved with Lady's servants that's all). But you probably know all that.
Finaly, learn to properly pronounce names of people and places (don't call Sigil "Sijil" etc...), becaouse Fetranity of order had imposed harsh penalties for such transgresions. It is on rang of 200$ fee for spitting chewing gum in Japan, only they also beat you and put you in jail if you cant pay. Bloody snobs. :x
Also dont bother Colectors, they are poor and homless who work for Dustmen and they colects corpses from strets during the night. It may seems it is grave robing, but it is unwriten rule (maybe Sigilian superstision) that they can keep all stuff they find on corpses. Think them as Eta from feudal Japan.
Trying to convert people is probably not viewed highly. Sodding primes who think that their gods are the only real gods and that everyone else should convert get really annoying really fast. Overzealous missionaries don't last long in Sigil, especially since quite a number of people don't worship any god (growing up in a god-free zone tends to produce quite a few skeptics).
@ lsdfjkdsf: Thanks for reminding me of this important factor: Churches and Temples. (aka giving me topic to rant about) 
Sigil is full of temples, wich is quite ironic becaouse there is only 15 factions and unnumerable temles and churhes of warious gods and panteons. Thing is: diffrence bettween planar church orders and that on prime is that palnar orders have replaced their faith (more or lesss) with politics. Faith is no longer point when thing you belive in (in this case your god) is just few portals away. So churches of specific pantheons hold as much political power as faction do, they just dont interfere with faction squablings.
So if you are dealing with clerics of Sigil bear in mind they are more politicians than men of faith and you could be draged in one of their plots, also don't make enemies out of them becaouse like I said: more poverfull servants of their bosses are just door or two away.
I always saw Sigil, as a harsh and unforgiving city. I see the planes like that but Moreso of the city of doors. As such anyone who has no skills or powers that they can exploit in some way soon finds themselves being crushed underfoot.
So a useless or talentless person in sigil doesn't last very long and is most likely looked down upon or ignored altogether.
One of Sigil's ironies is that everything must be imported, so wanton destruction of working portals is greatly frowned upon. Altering the exterior of a building in any way is frowned upon, with a no-net-loss-of-bounded-spaces rule. Don't let ayone catch you damaging one of the many alleyway arches or other pubic portals.
On a similar note, portals in public places are considered free for all, but the knowledge of their keys and schedules is not. Key shops exist throughout the city to serve planewalkers, but their fees are high and priced well out of 'casual purchase' range. Sharing knowledge of portal keys and locations is not only considered imprudent, but local key shop owners consider any dissemination of portal lore as competition. There's nothing wrong with selling knowlege, so long as a cutter's getting something for it, but a berk rattling his bone box for free has to be shut down, because she's messing with folks' livelyhoods. There are no laws against spilling the darks of portals, of course. But you won't make any friends giving away free meals right next to somebody's restaurant, and you might not get very good service the next time you eat out, know what I mean?
While portals in public spaces are considered free for all to use, there's nothing wrong with fortifiying the other side. A portal that's guarded in this way, out of courtiousy, usually has a guard just on the other side of the portal who politely turns interlopers back.
I remembered another one:
Don't speak against factions (at least not openly in public). You see factions have their agents (and even worse: fanatics and zealots) and they dont look too kindly towards people who are tarnishing their faction's reputations.
For example (that one is from factols manifesto): there was woman in Sigil, and she thougt it was funny to call the Dustmen: "the Dustbunnyes".
Untill she woke up one day and she find zombie in her bed.
It is rummored that zombie had rabbit ears, but no one belives that the Dustmen would have such sense for humor.
In general, it is a bad idea to take the name of the King of the Cross-Trade in vain. Business or normal social discourse is one thing, but foolish talk about the King tends to get back to her pointy ears.
Considering that the Harmonium patrols the city, I guess that it is wise not to say anything against them.
when they are near.