i do it probably with every quilt i make- i've never had it cause a problem- If i'm using a fabric (like a deep saturated color) or red i do check to make sure it's not going to run- if a fabric feels funky or smells funky i wash it- if that's not a problem i don't bother- if it's flannel (and not an all flannel quilt) i do pre-wash flannels since they do shrink more than regular cottons- but i've made great quilts that have calico's, batiks, flannels, solids and even felted wools, corduroy or velveteen all in the same quilt- when it's all done, quilted & bound i wash them---they have all lived through it- and made people very happy, won a few ribbons/prizes & brought me in some pretty good $$.
i pretty much only worry about colors running, or funky smells/feels. I think people can sometimes really get themselves 'caught up' in all the worries...what is the worst that could possibly happen? since backing and batting play just as much of a roll in shrinkage as the top fabrics... i have never had a quilt shrink in a (wonky) way- where one block did & the next didn't...i wash all of my finished quilts & i dry them generally in a normal cycle of the machines...even the wool ones. (the wools are pre-washed mainly because i felt them all so i can cut them up & use them how ever i want- so they are totally wash/dryable when done i often use flannels for background squares with wool appliques- that flannel is prewashed (most of the time)