It isn't unusual to have to resquare after several cuts. I don't think it's because you're doing something wrong. I fold twice and cut through 4 layers. I would think there's less chance of slippage when you cut 4 layers, simply because you aren't cutting such a long distance. I put the guidelines 4 quilting gripper strips on the bottoms of all of my rulers. I hated cutting before I got those; I could not keep the ruler from slipping.