Preoders usually aren't necessary. Even if a game does sell out on opening day (which is rare), you're usually looking at no longer than 3-5 days til the resupplies start hitting. So save your money. You can wait. Oh, unless it's Fallout 3. Because if you preorder it at GameStop, then you'll get this fancy little soundtrack sampler. It may be a good listen, it may not be, who cares! It looks great.

Fallout 3 soundtrack/poster for GameStop pre-orders [Bethesda]