i never pre-wash pre-cuts...they are not cut with the intension of being pre-washed- the manufacturers will tell you they do not expect them to be pre-washed- todays fabrics (supposedly) will not bleed (unless of course it's a hand-dye or batik)
and it is not recommended to pre-wash them...
some people always do- and then you get to deal with smaller pieces than you wanted to use. you can (make do) or you can try over- or you can wash the rest of them so they are all smaller- chances are they will not be consistant...
if you are going to pre-wash pre=cuts it helps to use a mesh bag- but that only helps keep the fraying down and keeps them from tangling- does nothing for the shrinkage.