We don't have a formal policy but it works out that almost everything that we use is Wizard's stuff. Currently, one player uses a number of spells from Relics & Rituals but that's about it.
We used a bit more in the past. One player has a psion, and we use to use a lot of the Malhavoc materials to supplement, but ever since the XPH came out we prefer to stick with just that. Also, my brother used to use some spells from some third party evocation book, but we haven't touched it since he moved away a few months ago.
I use a bit more third party stuff as GM, but it's mostly in the form of using adventures (or chunks of them) rather than adopting prestige classes and other kinds of rules modules.
I love Green Ronin's Book of Fiends and Necromancer's Tome of Horrors (both I & II), but I never seem to use them in my games. Lack of appropriate minis might be one reason for this.