I've never prewashed a thing, except when required for a swap.
Just finished Bonnie Hunter's Talkin' Turkey (red and neutral quilt). When the time comes for it to be washed, I might throw some color catchers in with it, just in case.
Starching (or Best Press) helps when I am doing small blocks - 365 blocks or anything 6" or less with lots of pieces (Farm Girl Vintage, for example). I find it helps with accuracy tiny pieces.
After the amount of loose thread trimming I did on Talkin' Turkey to get it ready to hand off to the LA, I'm thinking I might start starching everything as I use it.