Sometimes...?
I wash if it's a fabric likely to bleed or shrink. Flannel ALWAYS gets washed, sometimes more than once. If I intend to use a white/light fabric with darks, I'll wash it. If I wash one fabric, I wash all fabs for the same quilt. I'm leaning more towards washing than not, lately, but I used to not wash anything.
I will, because of my latest batting fiasco, be pre-shrinking my batting from now on.
I wash with detergent, but not fabric softener or dryer sheets.