Well, it depends on HOW I'm asked.
If a guy sends a clever, funny or thoughtful pm (and especially if he knows me from the forums and can comment on my posts), then I am more than happy to respond to his pm. I usually leave comments on his gallery even if I don't respond to the pm.
I don't bother to respond to PM's that are obviously multiple mailings or impersonal cut and paste jobs.
IMO, if a guy REALLY wants to ask a chick to rate his dick or comment on his gallery, he should approach it strategically with a little of the same care he would put into asking a girl out for a cup of coffee.
After all, he wants to get a response, doesn't he?