My method of squaring is similar to Tartan's, but I just use regular yardsticks between the corner square rulers. They're just for marking, not cutting, so they don't need to be acrylic. I mark the cutting line about ¾" beyond the edge of the quilt top all the way around and do a preliminary trim on that line to square it and reduce the bulk for adding binding.
Once the sandwich is square, I mark a line back in from the cut edge for the raw edge of my binding, making sure that line will put the stitching securely on the quilt top. That guarantees that the binding goes on squarely even if the edge of the pieced top is a bit off.
Once the binding is sewn on the front, I take the binding width from seam to fold, divide that number by 2, and cut that distance from the binding seam to the edge of the sandwich. That guarantees that the binding is full no matter what it's width with no more than ¼" seam taken on the pieced top itself (so no lost points with wide bindings).
It sounds like extra work, but the results are well worth it for me no matter what the furure purpose of the quilt. I treat them all the same cuz they all deserve the best I can do.