David,
I used to align my enlager with mirrors as well prior buying a Versalab Parallel. The main advantage of the Parallel is that you can check alignment in any position of the enlarger head without changing anything. If your enlager is perfectly aligned at one head position, it might need a correction for very large prints with the head in top position, depending on the precision and length of your axis. The Parallel is an "alignment on demand" tool and mirrors are not really an alternative for that.