although too low you may limit your consistancy and suffer drop off.....
basically if you are shooting as fast as you can with it at bottom and experience no drop off leave it there.
if you get a little drop of, increase it a few clicks at a time until you get rid of it.
the higher the Rate of Fire Delay, the longer the solenoid will wait between cycles (if you like)....thus the higher it is, the lower your actual BPS will become.
as Mat says, it should be pretty near bottom with a well tuned ion and that will not really stop you pushing towards 17bps.
if you need to shoot slower, eg: for a capped match, then up the ROFD until you slow to the desired bps - handy if you have one of those chronos around that measure both fps and bps