I don't have a panel saw, so usually when I cut up plywood sheets, I have to some times cut it with a straight edge, and a skill saw, and it doesn't end up square. If the panels are small enough, I can square the ends on my crosscut sled. if not this is what I do.
Here is my trick for squaring up wide panels.
I lower my blade then use my crosscut sled to hold one edge square.
then I bring my fence up close to the edge and lock it down, I put a couple three pieces of double sided tape on the panel.
Holding the panel to the crosscut sled and I hold my straight edge against the fence, and hold it in place with the tape.
I remove my sled, and slide the fence back to where I'm taking a small cut on the end of the panel then cut it holding the straight edge up against the fence.
Remove the straight edge, and tape adjust the fence to the correct length and cut the other end of the panel. I now have a square panel.
Panel to big to square up with crosscut sled.
Showing end out of square
Add double sided tape
Attach straight edge.
Set up cut using straight edge against fence.
Now end is square.