I got tired of this and started buying wide back solids by the bolt. That way I can rip the fabric off of the bolt myself - always comes out straight! For awhile I was asking shops to rip wide backing off of the bolts but I got tired of the looks of horror.
I still buy cute prints in wide backs too, but I buy lots of extra and then I rip the ends off when I get it home to get it square. I swear most of the fabric I buy these days are printed out of square and then wrapped onto the bolts in a less-than-straight fashion too. No wonder the staff can't get a straight cut!
Plus, it's just fun to rip fabric! I trim the ripped ends about 2 inches in, and fold & braid the strips into dog toys, so it's not really very wasteful.
