I always use up locking seats because it makes it a lot easier to balance the rod with a wider variety of reels, otherwise with a down locking seat you limit yourself to the heavier stuff to balance the setup, not mention it helps get the reel up and out of the way a little bit which provides a little more room to move your hand on the bottom grip. imho
Use whichever style you personally prefer. Me, I mount reels seats as downlocking on any spey (2-handed) rod I build for myself because it makes it easier to ballance the rod when strung-up with a line and fishing the swing.
I didn't give this any thought until my second spey rod build when I noticed that the reel seat from my first spey rod build 12 years earlier was installed in the downlocking position. Apparently, downlocking was how the seat from the factory rod was positioned and I just copied the factory rod - it was a Scott. That said, I do like how a rod balances with a downlocking seat and would build my next one downlocking if I could find the right seat (and needed another spey rod). One challenge to installing a spey seat in the downlocking position is that the metal retaining ring on the larger spey seats are quite large, so the cork needs to be very thick in order to properly cover the retaining ring.... which may not look all that great on a rear handle.