That woodpeckers mortice match is an impressive bit of engineering but at $650 not really an option for many and the offset base system is $470 so great engineering but aimed at what market.
Good point re board thicknesses. I guess I'd thought if u could centre it, i wouldn't have to think about faces....you rarely get boards where both faces are equally good so you want them to be correctly orientated, and if you are off centre by a couple of mm a mistake in cutting the slots or dry fitting is much more obvious than the slight difference provided by the almost (but never completely) centred slot