If you are still on the fence about adding borders to make it rectangular - lay the square quilt out someplace - I use my living room floor - and then I take fabric I would be adding as the extra borders and fold it to the proper size and arrange it around the quilt. Live with it for a little while and see what you think. This way it's just a matter of picking it up if you don't like it.
A quilting friend of mine does a lot of "auditioning" when she quilts and I have begun doing it on some projects. Sometimes what we think will work doesn't.