physically square, probably not, as most fabric gets stretched a bit off grain in the manufacturing and rolling process. However it will be straight of grain and that is what I go for. I always prewash my backing and find that helps a lot in relaxing the weave and getting to close to straight.
It does depend on what you LAer wants. I like to go not more than 10" bigger on the side I'm going to mount (usually the top, but sometimes I go sideways) and then I leave the length what ever it is and trim that off afterwards. I don't like having a lot of extra on a side when I'm advancing the quilt as it does not have batting so does not roll up the same. When I have that, I'll pin in a tuck occasionally as I roll to take up the slack.