lots of people make fabulous quilts-and never pre-wash anything- lots of others prewash everything- and insist if you don't your quilt will somehow be ruined-
pre-washing is a personal choice- reason to prewash are basically--
to remove chemicals/sizing
to pre-shrink
or to make sure something will not bleed-
so if you are not worried about your fabrics bleeding- the (stiffness) of the fabrics from the sizing does not bother you don't worry about it- your quilt will turn out just fine-
my grandma's flower garden probably contained an equal amount of pre-washed and not prewashed fabrics- it came out fine- it launders fine- there are no issues with it- it did not matter that some fabrics were and some were not- except for that one purple that ran a little bit the first time it was all washed as a finished quilt-