I would never ask anyone for a review. It's like a waiter or waitress asking for tips.
Like everyone has said, what if the service was bad? Then you are giving a false review and people will think, "Oh yeah, amazing service" And be totally disappointed when it's all done. Sure the girl is getting more money in her pocket, but it won't be for long because the men will find out it isn't worth it and stop going to her.
Reviews are earned and given out as deserved.