It's not assault to get in someone's face, even though some may tell you otherwise. If they touch you then you it's a different story but you need to make sure that they intend to cause you harm first.

That being said don't slap people if you're going to fight, you're going to get your head knocked off by anyone with any type of experience.

Second, I wouldn't rely on Jiu Jitsu in a fight, if anyone is around they're going to pull you off, or worse you'll get stomped out by all their friends. Or maybe you get unlucky, hit a takedown, and bounce your forehead off a chair on the way down.

Learn to box before you go picking fights with people over nothing, unless you enjoy getting KO'd.

Personally I just turn my back on people who want to talk shit and walk away, when they follow you to try and get the attention they so desperately crave, turn around, post their throat (if they're right there, or wait for the inevitable big man strut to you as you turn around), and knock them the fuck out.