Sounds like a recurve question but not necessarily. With compound, the peep gives string alignment left/right and up/down in one hit.With strings and no peep you get left/right alignment. I brought the string blur as close to the sight as possible without blurring the target image. It then became part of the "picture" without having to check two aspects separately.
Forgetting to use it is not really the issue, I feel. If you forgot but it was in the same position every time;no problem. If you forgot and sometimes it was further left or right than normal; then that is a problem. The string picture is the rifle equivalent of the back sight as far as left/right is concerned.