I prewash, mostly because of the chemicals used to preserve the fabric and to catch any bleeding colors (I use a color catcher in the wash) and to preshrink the fabric (some fabric shrinks, some doesn't but I want to be covered either way).
I do not prewash jelly rolls though, or anything precut in small or narrow pieces. It's just too much, trying to untangle them if they fray a bit and then I have to iron every little piece so I just cut and sew and don't worry about it. So far haven't had any problems.