I think sometimes its ok to ask, but it all depends on how you say it.
For example, I hooked up with this dutch chick a few months ago. We were at a concert and we took a breather and just sitting down having a chat and I could tell she was interested so I just told her 'I really want to kiss you'. And of course she did...
I think the mystery kiss Q is good also. Ask a girl if she would like to kiss you. If she says yes, do it, if shes maybe, say lets find out and go for her. If she says no. Say I didn't say you could, I was just asking.... haha
So all in all, like the other boys said, if you can avoid asking and just do it, it will probably flow a bit easier, but if your 50/50 I don't think there is problem asking.
One great line I used once was, I had just meet this girl at a day party. We ended up going out for dinner and she was very talkative. So after dinner we walked down by the harbor and she was talking and I just stopped her mid sentence and said 'You talk to much', then started making out with her. She was thrilled and shocked at the same time. Thrilled because she liked me, shocked cause it was completely unexpected which definitely added to the excitement.