I would say either prewash it all or don't prewash any of it. However, prewashing is a very personal thing. Many panels are very distorted when they come off the bolt. Wetting them can allow them to return to their original shape. Wetting them can also make cotton and especially flannelette distort as they dry. Yes, it is all very confusing.
I have had good luck with 'rinsing' my fabrics in as hot water as my hands will take, squeezing, not wringing, and then mostly drying in the dryer. That allows me to see any colour bleeds and provides the opportunity for shrinkage, as well as getting rid of a lot of the sizing that manufacturers use.
If the panel is significantly warped in shape, I will tape down two sides, dampen and gently (or aggressively on occasion) coax the other two sides into uniformity, continue to tape all around, wet fairly thoroughly, and let dry. It usually solves the problem.