You can pretty much guarantee that flannelette WILL shrink!
Because of the looser weaves of flannelette, thus the shrinkage.
I read something recently, that you can expect at least 5% shrink.
That would meanfor 40" WOF, a 5% shrinkage would be two inches.
I've had it shrink more on repeated washings.
Quilting cottos , some shrink, some not so much.
So there is less risk by not pre-washing, though there is a risk.
Though if one shrinks a lot, and the fabric next to it doesn't ... what do you have?
As for fabric bleeding ... quilting cottons and flannelettes are all susceptable.
Washing them once, does not mean you have dealt with that issue.