I'm a prewasher too, for exactly this reason. Almost all of my fabric except for some fat quarters which I am thinking I need to wash now, comes in the house and in the machine...usually on hot...and in a hot dryer, too.
I might be less adamant about it if I only bought fabric from fabric stores, but I get mine at thrift/yard/estate sales. No way do I trust it until I've cleaned it!