I found fabrics do indeed shrink differently. I hate to preWash but especially with darker fabrics I am afraid not to. Some fabrics are just woven tighter/looser and if you plan to mix in batiks I find they don't shrink much at all.. Guess it is just personal preference and how "picky" you are with the consistency of the 'crinkling' on your final result.. If you deal with flannel you can get HUGE shrinking differences, that is the one fabric type I would say you should always pre-wash!