I tend to be selective about what I use. Primarily, I'm more into race-types with Class/Levels. Monster encounters (except for specific campaign regions) are often the rarity, particularly the big and nasty ones (after all, what's a campaign without dire rats?). As such, I often only include monsters that are most fitting to the situation and the environment.
The result, of course, is that I've probably only used about 5% of the monsters available. This includes MM, MM2, CC, CC2, Tome of Horrors, MotP, ELH (Glooms! Gotta love them Glooms!), D&M, D&M2, Bestiary of Loerum, Monsters o/Faerun, BoVD, OA, Creatures of Rokugan, and a bunch of odds and ends gleamed from the multitude of sources (Fantasy Flight Adventures, AEG's one-word title series, etc.).
If it inspires an encounter, I use it. If it creates its own niche, so much the better.
By contrast, however, I never pass up a chance to buy a new monster book (although I passed on Minions and Monsternomicon so my DM can actually surprise me with some new stuff in our upcoming Oathbound Campaign). While only a few things have inspired me enough to use, I'm always on the look-out for new things (for instance, the new book coming from Inner Circle Games looks kewl, and there's one by Fantasy Flight (I think) that I'll be picking up soon). I even picked up NG's Necropolis just because it had a killer monster section.