I always wash mine for several reasons. First, I am not a prewasher of fabric, second, I know they drag or fall on the floor sometimes, third, I use Elmer's Washable School Glue for basting, and fourth, I use cotton batting and know it will look different after it is washed and don't want the recipient to think there is something wrong with it.
Regarding knitting, my mother knitted a lot and she always washed and blocked everything she finished before it went to anyone.
I don't wash new clothes before I wear them. I've never had a problem and am not sensitive to dyes or sizing, I guess.
Glad to know there are options to make everyone comfortable!!