Cool Tool

Quite frankly if your quilt isn't going to hang but just be draped over a sofa or a bed this isn't too important. If you are hanging a quilt in a show or on a wall squarely it matters. It matters because if the corners are not at 90º, if it's big it will create slight folds and if it's small it will annoy you like a painting which is slightly off kilter.
This is a laser tool made for tilers. It gives them exact 90º angles to work off. To use it for quilting, lightly stretch your quilted top and secure with pins to your carpet, or masking tape to a solid floor. Place you laser tool in one corner and make little marks on the laser line about 12"-18" apart on the two lines. Then move the tool to the next corner, line up with the little marks you have made on one side, and mark the next. Then do this to the next side. Then go to the final corner and check that your very first row of marks and your last one line up.
Then just take the quilt to your cutting table and rotary cut between each of the marks. This sounds like lots of steps but it is far easier than trying to slide rulers side by side to get long straight lines (and in my experience, they always slide a little out of kilter).

This isn't going to give you a cast iron guarantee your quilt will hang straight, there are other factors like balancing out the quilting which will affect it, but it does give it the very best chance (grin).
I like to do this at the same time as I block the quilt before adding the binding.
You can pay a fortune in DIY stores for a laser but I got this one on Ebay for about £15 ($US30). You could just about manage with one which has only one laser and use a ruler to line up your 90º, but two lines are so much better. On this particular model the lines are adjustable so if I wanted I could with a bit of fiddling about get 60º and 120º as well, though I'm not sure I'd ever need it.
And be very very careful not to zap eyes, get it in position before turning it on, and make sure no animals, children, or men are in the room. Men seem to be very cavalier about picking these up to have a look at them.