Just as a note-- many battings also shrink a bit. I always read the label when I am deciding which batting to use in which quilt. Sometimes I want a puckery antiquey look, like on my reproduction quilts, so I want the batting to shrink up a bit.
On my favorite battings, these are the shrinkage rates.
Hobba 80/20 shrinks about 3% after washing, warm or cool.
Quilters Dream 100% cotton shrinks 1% in a cold wash and 3% in a warm wash.
Quilters Dream poly shrinks only in warm wash, 1%.
Ironically. Quilters Dream Wool doesn't shrink. Go figure.