My reason for pre-washing is to try and minimize the crinkle effect that comes from the batting shrinking after quilting. This bat measured the same width before washing as after, but was two inches shorter. If most bats do have wavy edges to begin with, then I guess there was no harm done in this case. I suppose I'd have to make two quilts using the same batting, one pre-washed and one not, to see if it makes enough difference to be worth the effort.
Thanks for your replies!