as for the old cans you have...test them on something small...they should be fine, but better safe than sorry. they may not (spray) so well anymore, ya know...spit blobs...so try them out before just pitching.
and, as far as the other questions...
if you are making a quilt someone else is going to wash...don't you think you should be the first one to wash it? just to make sure it comes out the way it should? i never let a quilt go that i have not washed after i finished it. unless it's a wall hanging not meant to be washed. that way if anything runs, or a seam comes loose, or whatever may happen happens to me...not the person who's going to freak right out..and i can remedy any issue before it goes. so if i use a basting spray (i don't) i would wash it out before the quilt goes onto a bed or out the door.
i seldom prewash my fabrics, but i ALWAYS wash a finished quilt, in what ever way i think the person getting the quilt is going to care for it. if it's for a kid, it's going to get washed alot...i want to know it's going to go through the wash and dryer ok before i give it to a kid who's going to be heart broken if something happens to it.