On a dance floor, or in a club I totally agree with the wrist grabbing. I also like to twirl girls, they seem to love it when a man spins them around. I'm a decent swing dancer, so can do some funky ones.
But during the daytime on a busy street...not sure i got the balls to just grab a girls wrist then, she might scream bloody murder and get me arrested for molesting her or something
But I really recommend not thinking of the perfect moment or anything like that, just approach soon as you see a person you want to talk to. If time is spent fidgeting, you'll just create anxiety in yourself and your delivery will probably suffer from it(At least that's how it is for me)