I don't for some reason, even with maniac drivers (which most of them are). dunno why, its a seinfeld bit I know but kind of weird.
Always unless I'm crammed in the back with three or four people, as happens sometimes when the bars clear out.
in the middle east and north africa, it's considered a personal insult to the taxi driver if you put your seatbelt on. some of the cabs don't even have them. and don't get me started on the traffic, and how they drive. I'd feel safer sitting on the roof of an American cab