it is a personal choice- if you have a hand-dyed fabric or a batik that is very saturated color (deep purple, red, blue ect) you may want to at least test it to see if its' going to run or not- (wet it & rub with a paper towel or white piece of muslin-see if color comes off)
if it's flannel it will shrink more than other cottons- so if you are mixing fabrics- using flannel & regular cottons in the same project you will want to pre-wash the flannel- if you are using flannel for a raggy quilt don't bother-
if you simply not sure- make yourself 2 small quilts- one with pre-washed fabric- one with not washed fabric & compare the outcome- see what you like-
i personally prewash if...i am concerned about a color running, if it feels funky- if it smells funky- if it is stiff & i want to soften it up-
if none of the above are an issue i may or may not bother-
i ALWAYS wash my finished project as soon as the binding is finished- no matter what- doesn't matter if the fabrics have been pre-washed or not- last step is to launder the finished project.