I wouldn't go below 80mm personally, and not 90mm ideally. You go too short and it's just bad news for handling IMO.

From your user name I'm guessing you do tricks more than actual riding around the city. But if you ride around the city and want a more upright position, I'd suggest keeping your stem and getting some "Priest" bars or 3 speed cruiser bars. Personally I think one should just man up and learn how to ride bullhorns or track drops in the city (which is perfectly do-able if you don't have a ridiculous saddle-to-bar drop and actually use the drops).