Clamping rail and stile joints closed doesn’t automatically guarantee that the door you made is perfectly square. I can tell you from (bad) experience that it’s very possible to assemble a door out of square. You don’t want to go there.
Get your door glued and in clamps, Photo 1. Check the door for square by measuring corner to corner, both ways, to check the diagonals, Photo 2. If the measurement is the same across both diagonals, your door is square. Congratulations! More often than not the door will require a little tweaking.
The easiest way to square the door is by making the clamps do the work. Here’s how to fix a door that’s not square: Start by loosening the clamps. Then rack the clamps, slightly, in the same direction as the longer of the two diagonals, Photo 3. As you gently retighten the clamps, they’ll pull the long diagonal shorter. Recheck the measurements across the diagonals and work the clamps until you’ve got the door dialed in, perfectly square.