I have learned to check the back of a print to see if it is printed on-grain or not.
I have also decided that if it is printed off-grain, then I have two choices - cut the fabric off-grain to use the pattern/print/design the way I want to - or not buy it.
Tugging it, blocking it, washing it, etc etc etc - it will still be printed off-grain.
As far as what quilt shops do - I don't know. I think it is "buyer beware"
I had a sales clerk tell me that a fabric that was printed off-grain would straighten out when it was washed.
Some associates are more knowledgeable than others!!!!!!