A lot of its going to depend on whether or not the die crimps or not.
If the die does not crimp (like a Lee dead length seater) set the seating die up so that its in firm contact with the shell holder. As long as the shell holder is pressing against the bottom of the die at the top of the stroke, everything should be fairly consistant. If your seating die is similar to the standard RCBS seating die where its backed out a turn or so from the shell holder so that it doesn't crimp, all bets are off.
If you have a single stage press, I would use it for seating anyhow. You'll want to use flat pieces of metal of consistant thickness (washers) to square up your seating die to the ram before locking it down.
I'm glad someone brought this up as I just got a cast turret press a few weeks ago and have been banging out a lot of cast rifle loads since. I haven't thought about checking OAL as that isn't too critical for me as I'm soft seating my bullets. The turret does have a bit of slop in it, on the other hand a single stage press that is not correctly set up might give you consistant oal, but could give you terrible runout, which is worse.