my favorite flannels- and i use alot of flannels- are
Benartex double sided flannel and Moda woolies flannels.
i love the heavy-double sided flannel- seldom waste my money on thin single sided flannel.
it depends on what i am doing if i pre-wash or not-
if making a raggy quilt i do not prewash-
if piecing- sometimes i do, usually if using dark colors (reds/blues/blacks) that i think may run a bit-
but more times than not i do not prewash my flannels.
always use a sharp-new blade in your rotory cutter & a new-sharp needle in your sewing machine- if i have to cut small pieces- or long narrow strips i will starch & press before cutting- but generally just a spritz of water is enough when pressing flannels.
*i am one of those-never put water in my iron- i keep a spray bottle of water on the end of my ironing board to spritz when needed*
i've been sewing with flannels for 40+ years-
i do agree with the comment about flannels from (certain stores) the thinner- and single sided flannels tend to (pill- get little bumps all over the surface) after being laundered- and they do not tend to hold up as well as a good quality double sided flannel.
flannels are a looser weave than regular cottons- and do shrink more than other cottons- but batting and quilting does also contribute to the shrinkage outcome of a quilt.