"We recommend only selecting two expansion packs at a time"

That is why I always keep all my EPs and SPs checked, because sometimes you can't even play a certain game save if you don't. The only time I will go in without anything unchecked is when I first start up a world. This way all those lots that came out in later EPs won't be there, and I don't have to deal with going in and removing them, just to place then where and how I want. This is specially nice with the junk yards, those things are always causing thing to spawn on the old lots (can't even find them using the debug cheat).