Thilo, the mirrors also work at any head position. I usually check all stages at the beginning of a major session, and then make quick checks when I move the head. It's the same procedure as the Versalab- a mirror on the bottom of the lens (without changing anything) and another on the baseboard. I don't do enough enlargements past 8x12 from 35mm to justify the $190 for the Versalab.
Either system is light years ahead of spirit levels, rulers and scratching X's on negatives.