When I DM, I set the limits up front. [Typically Core + XPH + Modified MotP + Draconomicon + Complete series but no CPSi] I limit to this because these are the books I own and these are the books whose rules am intimate with. I don't like to DM using sources not available to my extended viewing pleasure whenever I want them. But that rule is established up front.
I also give the players the option of adding a source when I purchase a new one. If the players veto it, then I don't get to use it as aDM, either. If they embrace it, then its all cool. I also allow the rebuilding rules from PHB II to allow 1 change everythime they level, so adding new books can still be cool even if the characters could have benefitted more by something in the new book. They can rebuild their character in stages to see it fit what they want.
But then again, I don't complain about too many options. I personally don't like the Races of Books at all - or the Environment books like Stormwrack, etc. But I don't use them, either. And I don't complain if someone else does!