If I'm certain that the project I'm making is NEVER (!) going to need washing (wallhanging) then I make it with unwashed fabrics just because I like the crisp look for "modern" decor wallhangings.
If I want the thing to look vintage or "prairie" then I sew first and wash later to achieve the "pucker factor," even if it is going on the wall or stair rail.
If it's something I'm actually going to use (bedquilt, lapquilt, placemats, clothing, toys, picnic) then I prewash for 3 reasons: the contact of the unwashed fabric chemicals on the skin is sometimes irritating to some people; stains sometimes "set" worse in unwashed fabrics; and it just makes the fabric softer and feels so lovely!
I started off my quilting career prewashing every bit of fabric I purchased, serging the raw edges to prevent fraying, but if your stash is anything like mine, that became waaay too much work reeeal fast!