I don't worry about squaring it up. I line mine up with the edge of the table leaving 2-3" over hang and start pinning. I no longer start pinning in the center. After many, many quilts I find this to be less wasteful. After pinning I trim all sides leaving 2-3". This makes it easier for me to quilt on my 9" throat machine.

I tear mine as well.

when I was quilting on my DSM, I never squared at all, but the long arm won't play nice without it being square--it's like a bad foundation on a house.

I have found that the wide backings can be off that much when squaring up. I think it's because of the size and it's hard for them to print it perfectly straight. It sure is nice though not to have that seam when longarmming.

I tear mine, too. I used to try to cut it and always had problems. When I heard about tearing it, I tried it and found that it solved my problem. Because it is so off I always buy extra.