It depends on the look I want for individual projects. If it's on a large quilt, I usually do double fold binding, pieced and joined on the diagonal. If I want to use all the pieces from a scrap quilt, I do it with straight seams to mimic the piecing in the quilt. I press the seam allowances open to reduce the bulk. I reduce the seam allowance a smidge over the joins because the joins are a little more difficule to fold to the back(you need a smidge more fabric to fold over) I also have used straight joins on small pieces like a mug rug because there isn't enough room to do a bias join with double fold binding. Do whatever join you want for the "look" you are going for. The only time it might concern me is if it was going into competition.