I use my sewing table (lucky enough to have a generous dining table size). I tape down one selvedge edge and one long side, tugging as needed and even dampening if it is really skewed. Then I square up the third side again tugging, dampening, taping as I go and finally the last side. Spray the whole thing with water and let dry. I find this is the easiest way. I have wondered about trimming because the panel could try to revert to its proper form when washed and then the quilt would be out of whack.