How I deal with panels -
1) Before purchasing, check to see if it is printed "on-grain" -
Of course, I learned to do this by buying off-grain panels
2) Wash the fabric and let it relax -
For me, it's just easier to let it "do it's thing" than to try to fight with it
3) I prefer to have things on the straight of grain, so I would trim it to be as "squared" as possible, and then add borders as necessary to make it work.
I am of the (strong) opinion, that if something is printed off grain - no amount of tugging, blocking, starching, is going to get it to be printed off-grain. One can compensate for it in various ways - but if it's printed off-grian, it is printed off-grain.