Where do you draw the line and decide something's too mean to say? Or do you just try it anyway? and if the reaction is negative, then what?

If you've gotten past this hangup, how did you do it?

I passed up some opportunities recently for teasing that I thought were going too far. For example, a girl complained about how flat her face is (she's asian) and though I teased her about other things I never teased her about her "flat face". I wasn't comfortable with that joke. She's lost interest, and I'm quite sure it's mostly because I was (still!) too nice.

TL;DR: Would you tease a girl about something she's clearly insecure about? Where do you stop?