Most quilts I use pre-printed squares on are for children, so the wonky look works. I agree with the others. No pre-printed squares are ever square. If you insist on square, trim them so the border is within your 1/4 " seam & make your own borders. Cute fabric!

Seeing as how you've already cut the squares -

I would wet the fabrics - (let them soak for about ten minutes in a bowl of water) - roll them in a towel to blot out excess moisture - and lay them flat on a towel or on a drying rack - let the fabric 'do its thing'

And then go for the compensating tactics.

The grain is going to do its thing - If you try to force it to the print - it might 'hang funny'

Like jeans that are cut off grain - the crease never goes quite 'right'

I would also love to know why they always border these panel prints!! This isn't the first time I have worked with a panel print, but it's definitely the worst I have ever seen. I did plunge it into water, rolled it in a towel and let it dry naturally. It is better but NOT perfect. I do agree with cutting off the border and sashing it all around, that's most likely the only thing that will be salvageable at this point!
I am so greatful to all of you for your wonderful hints and tips!! and... your sympathy! giggle I know I can always count on getting help from this wonderful group!
Thank you sew very much and when it's finished I'll post a pic of my trying NOT to be wonky, wonky wallhanging!

Thanks, Prism99, for the Sharon Schamber video links! They are really good!

I've learned - by buying crappy panels - to check to see if the 'squares' are printed more or less on grain before buying the panels now.

I'm sure the other shoppers think I'm strange folding the fabric back and scrutinizing the weave, but - oh, well - I'm old(er) now and have gotten a LOT more selective.

I had 4 large panels that were not square and HAD to be because I was fracturing them. I basically beat them into submission!!

Wet them thoroughly with water/starch mix. Pin them out on a grid and force them into square. Then ... wait for them to dry.

My panels were about 2' wide and 3.5' long and I drew a grid on my carpet (it's those carpet squares with a very very low "loft" - almost flat) with chalk. I first pulled on opposite corners of the panel to stretch it into shape as much as I could, then I then used the grid to insert pins through the fabric and into the carpet to hold it in place - working the shortest opposite corners first and working my way evenly across the horizontal and vertical straights. I used LOTS of pins.

It did work but word of caution ... do not handle them too much once you pick them up. Do not iron them again. Sew them immediately.

I have purchased panels in the past and NEVER could get them to square up. Think I am done with panels.

I would just add a border, then square it.

I'd give up on it. Or give up squaring since the pattern was printed off grainline, you'll NEVER be able to get it square.
I'd had the same problem.
SandyQuilter