For saddle centering, that works if:

1. Seat post markings are perfectly centered.
2. Seat tube slot is perfectly centered.
3. Saddle rails are perfectly aligned.

I'm yet to see a combo where all three are true. Don't forget either that misalignments at the seatpost get amplified at the saddle, so even if all three conditions are met it can be quite hard to center the markings in the slot well enough for the saddle to end up perfectly centered.

For saddle height it's OK, but if you're removing the seatpost often and want to always have it at the exact same height, it might better idea to get a clamp that goes on the seatpost. See for example the BBB Postfix.