I pre-wash everything for two reasons: So I don't have to deal with any chemicals the fabric has been treated with or any soiling that has occurred in passage, and because I prefer the idea of any bleeding or shrinkage happening before stitching. I agree with the notion stated above, that if their are reasons to wash a larger piece of fabric, those same reasons apply to a smaller piece of fabric.
I've worked with jelly rolls twice. I did wash them. I washed them by hand, spun them in mesh bags in the dryer, and hung on the line to dry. Fabric shrinks much more over its width than over its length (just like it's much stretchier over its width than over its length). Since jelly rolls are cut across the width of fabric, they shrink much more over their length (the width of fabric) than over their width (the length of fabric). So those jelly rolls I washed shrunk a bit in width, but not enough to prevent me from using them as 2.5" strips (I think one set was pinked and the other wasn't). I didn't bother checking to see how much they'd shrunk over their length.