Well, right or wrong, I usually cut mine on the cross wise grain - from selvage to selvage. There is a tiny more stretch in the cross wise and sometimes it is helpful since when the pieced blocks are sewn together the measurement might be off a little bit. So the little bit of stretch in the crosswise cut can be beneficial. And you are correct, if you cut your fabric on the diagonal - 45 degrees on the selvage - that is bias. There is a lot of stretch in a bias cut.