I do the same as hopetoquilt says.

On wonky panels stretch them while they are wet and pin them in place. That usually works.

I just did this with a flannel panel that was approx. 23" x 42" and it came out great. I squared it by pinning it to my ironing board (24" x 48"), sprayed it with water until wet, patted it into shape, let dry and repeated with a spray of Easy Press. When dry it was perfectly square. I then set my 12 1/2" ruler in the corner and extended the sides by placing my rulers end to end to the length of the panel. Now I can frame the panel with the blocks I made and the whole quilt will be square. Hope this helps.

I wash/dry the panel, then spray with a mixture of 4 parts water to 1 part vinegar. Gets the wrinkles out. If it's tiny panels, like the morrison kitty panels, they were just an 1/8" longer than wide so I use the Hole in the Barn Door quilt block pattern then it was not noticeable that the block was longer. If it's a long panel that you want to use in the center of the quilt like a medallion, then I add a wide border to it then trim it to a workable dimension like divisible by 2 or 3. Makes it easier to use block patterns after that. I just did one that that's what I did. It winds up framing the panel.

My 4' T-square from the hardware store was one of the best investments ever made for my sewing room. About $20 dollars at Lowes or Home Depot. I also use it to mark my batting to cut from the roll.

If you pull it, and stretch it and press it to make it square. . . . what happens after it's done? The first time you wash it? Does it get all wonky again and make your quilt look really bad??

Quilting .... will hold it in place!
Of course, you can't expect 4" apart to hold it together .....