I was talking about sex with a friend (female) the other day who let on that it absolutely creeps her out whenever a guy "asked" for a BJ. She wanted him to act and sound like a man and "tell" her to go down, or have her positioned down there.
I thought about that and tried to imagine asking for it--I couldn't do it without a sense of begging <shudder> or feeling neutered, to a point <another violent shudder>.
They want you to know how to handle yourself in bed.
They want you to know how to handle them in bed.
They want you to show confidence--and DOMINANCE, even.
I'm not saying being considerate and having open communication is to be chucked out the window, but somehow "asking" just seriously undercuts everything they want you to be in bed.
And creeps them out.
I'd suggest you don't make it an obvious exchange of "I'll do this if you do that" thing, even if unstated.
Be active about it, it shows confidence to you and her. Wait till the foreplay gets steamy and direct her down there, or get up into a standing position and bring her to you. Massage her hair and the back of her kneck while she's going down, and let her HEAR from you how well she's doing--talk, moan, call her name, tell her what effect it's having, let her feel you tense up.
If she likes being dominated simply say "go down on me" like a command. You won't know until you try--by all means, don't ask her just yet--save that conversation for later. She may **think** she doesn't like to be dominated and tell you so, but how she will respond can be entirely different. Her preconceptions may get in the way--which is why you ACT, don't ASK.
Hope that helps...