Do whatever you want, but just do it consistently. Having half your stash washed and the other half unwashed will result in problems when you forget whether washed or not.
I don't prewash unless I'm making swap stuff. The only disaster I ever had was from red flannel that WAS prewashed and two Jenny Beyer fabrics from her very early lines. I still use the quilt despite its stains on the front. Not a big deal to me, but not giftable.