Now I (usually) check the fabric to see if it is on- grain before I buy it. Sometimes I do forget to check.
I wash and dry the fabric to see if it will go back to on-grain. Sometimes that does the trick.
If not, then to the diagonal pull and tug on the fabric to attempt to straighten it.
Then, after getting it more or less straight, I would resort to heavy starching to reinforce the new alignment of the threads.
If the design is printed off-grain, then the choices are cutting off-grain to follow the design, or cutting on-grain and not following the design. There is no pulling a design that is printed off- grain straight.
I still prefer to have my pieces on- grain - I think the finished block is a lot easier to press then.