I don't think this is entirely fair. Yes, if you expect the Dom/me to gratify you the way you want when you want, then you are topping from the bottom, and BTW, since you know this and feel so strongly about it, I wonder why you started this thread as a question? But there are times, say pushing limits, when it's perfectly reasonable for the sub to just say no, but it might be more enjoyable for everyone if s/he gave it a shot. And at those times, s/he might have some concerns that the DOM/ME CAN'T READ HIR MIND ABOUT! Maybe it makes the difference between a good scene and bad one, or a good scene or no scene. Maybe the sub would like to modify things a bit, in order to be able to participate. And maybe the Dom/me would prefer to know that rather than have the sub exercise hir right of refusal.
So, I repeat my question to Dom/mes that fall into this category. Dom/mes who prefer a miserable sub or one who won't try new stuff, move on. The rest of you, though, could you please tell me how you allow your sub to help you shift things during a scene? Obviously, it's better if you've discussed things beforehand but that doesn't always work out for a variety of reasons.