Back when I cut mine in a 1/2" crew cut, I preferred to have a straight line across the bottom, but with as short as I keep it now, it doesn't matter. What I used to do back then was take a piece of wide masking tape and hold it behind my head so the I could even it up from the front without looking at the rear of it and then place the masking tape on my head/neck. I would then go by feel and put the edge of the clippers on the tape and cut downwards to get my neck. That gave me about 1.5" of protected area along the bottom of the hair to keep me from cutting too far up.