If you’re looking to square a case, clamping from corner to corner is OK if you’re just worried about the front edge of the case. To really square the case you need to check it from front to back using a square.
How you set and adjust your clamps is key to attaining a square case. By racking the clamps – a clamp head higher or lower than its tail – you easily can finesse the case into square.
If you look at the photo, you can see how the clamps do not align with the drawer blades, which are level across the case. The top front clamp is adjusted a bit; its head is slightly lower than the opposite end. The lower front clamp is adjusted significantly. Clamps at the back of the case work the same.
Clamps at the bottom of the case do not move the case into or out of square the same way as do the front and back clamps. Bottom clamps (clamps across the top, too) need to be pulled away from the case to have an effect – move the head or end so that less of the clamp pad is on the project to dial-in square.
You need to experiment a little to get the hang of it. As you become experienced, you’ll quit adding a clamp across the corners to bring the case square, and begin to use your clamps to square the entire project.