have made 5 quilts with jelly rolls, in the middle of #6..have never prewashed them and have never had a problem, even a lecien one which had a few burgundys in it..
moda fabric is very high quality patchwork fabric, you should never have to prewash it unless you truly are worried about a particular red, purple or blue..if thats the case, snip a weeny corner from any of concern in the range, wet it and put it on a white tissue to dry..no bleed its fine, as all in the range will be..
rule of thumb, any high quality patchwork fabric shouldn't shrink enough to affect any quilt top..if in doubt do 2 things:-
1 - look closely at its surface and run your fingers across it, it should look even and feel smooth..
2 - hold it up to the light..the bigger the spaces between the weave, the more chance of it shrinking..if the spaces in the weave are near as wide as the threads in the weave, or wider, prewash..usually the good stuff doesn't have this problem..but don't let it stop you buying a bargain either, just allow extra when you buy and prewash it before adding to the stash..
hope this helps, huggs, pami