I did mastering for vinyl in the late 70s to early 80s and MONO BASS was almost always done during mastering. It is primarily used if there's too much low frequency information on either side, left or right, as that would create too much lateral movement of the cutter head and could easily throw the playback stylus out of the groove!! Making everything MONO below 100Hz or so would solve that problem.
All that to say it's nice that RME gave you the option, but for most releases done after the late 70s, it's not necessary.
_____________________
Eric Seaberg • San Diego, CA • USA
A.E.S. • S.M.P.T.E. • S.P.A.R.S. • I.E.E.E.