NowDNAtoo:I put a waterproof one on the mattress. This way I know we aren't going to end up with stains on the mattress, especially since it needs to last through toddlerhood.
we're definitely doing this (in fact, the pad is already on our mattress). i've heard way to many horror stories about diaper blowouts, puking, etc. to trust a thin little sheet to protect our $200 mattress.
Ditto. I have mattress pads on all of our beds, so it never occured to me to not buy one for the crib mattress.
We used it religiously when DS was a newborn. He had lots of leaking and blowout problems. We also used it in the winter, because the mattress gets a little cold, so it provided a barrier between the mattress and the sheet.
We haven't been using it now that it is warmer. DS doesn't really have leaking problems anymore.