I wash and dry - preferably in a dryer - fabric before cutting it - to see what it wants to do "on its own."
I have found that most blocks (assuming they are going to finish up as a square or rectangle) press a lot flatter when the individual components in the block are "on grain" - or the threads are perpendicular or parallel with the edges of the block.
If I tear - which is not often - I am prepared to sacrifice the shredded ends -
If a straight grain is very important for something or I need to be very frugal with my fabric - I pull a thread - which is time-consuming, but a fairly good "in front of the TV" project.
As far as panels go - I have found some that are fairly close to being printed "on-grain." I have learned to look at the back of most fabrics to check grain ines before I buy them now. If they are printed way off-grain, one can pull an tug on the fabric from now until next year - and it will STILL be printed off-grain.
I have also learned that some fabrics will "straighten out" when washed.
Some will not. I learned that one when an employee at a LQS INSISTED that an Alexander Henry fabric would straighten out when it was washed. It did not. And half of the width was extremely wonky. It was still wonky after I washed it.