bkay

Laura (?) of SewVeryEasy has done a couple of videos on straightening panels. She did one several years ago that she showed how to disguise the crookedness. The panels featured the comic strip "Pickles".
bk

Onebyone

I have lots of fabric panels I stretch the panels to square up. Some I cut into big squares and use sashings like attic window style for the back of a quilt.

Fizzle

Donna Jordan of Jordans Fabrics has a great video on straightening a panel. Also some fun ways to cut up in the attic style like onebyone mentions.

Karamarie

I like working with panels. If they need straightening, I can usually either trim a little and redo the rest to be the same. Maybe I have just been lucky when buying and using panels. Good luck!