I have learned to check to see if the panel if printed on-grain before buying it.
If the design is printed off-grain - you can stretch the fabric, pull on it, block it - stand on your head - the design will still be printed off-grain.
You basically have two choices when attempting to work with a panel that is printed off-grain -
1) keep the grain lines perpendicular and do something with the design - trim it, sash it, whatever
2) get the design "right" - and live with off-grain fabric that will revert to it's "normal" unless stabilized some way.