I think it's worth asking. Why waste your time or hers (not to mention emotions), if unvirginity is going to be a deal breaker?
I remember once I left it too late to ask (I just presumed, thinking it was like bible times) - it must've been the fourth or fifth date when I found out, and she was upset when I broke up afterward. Probably I was naive, but these days, there's no depths of depravity that people won't plumb and think it's totally normal (murdering kids is another one, but the virginity question addresses this too - a woman can't murder kids in her womb if her womb is unused/undefiled). As someone pointed out, these days, it's probably even worth checking she was born a girl. Confirmation of virginity is relatively minor in comparison to some of the necessary checks.