I always seemed to have problems getting any hems straight. I found a neat little notion that is described at a Button Hole Spacer. I think I paid about $3 for it at Jo-Ann's.
When I'm ready to hem, I set this little slide on the ruler to the depth of the hem. Then I just go around using this and pin the fabric. I have also measured twice the depth and marked the fabric and then just sew by aligning the edge with the mark.