I've always marked all my screens center horizontally and vertically on the frame (screen side) with big "V's" pointing inwards. Mark the positives center similarly and eyeball it in the dark before exposure with the help of a T-square then tape the leading edge with some transparent tape.
For me, taking a few minutes to line up the image before burning makes registration a little easier when printing. I like the idea of marking the centres on the 'styles and rails' of the screen frames. Good idea Thornysarus!