mono switch- off or on?


Kind of a dumb question but i'm curious-I have an AR PH5 pre with a mono switch. Purchasing a second tonearm and mono cartridge for dedicated mono. Any harm in leaving the mono switch on or should it be shut off when using the mono arm and cart?
slvrtravel
off.
with it on it will mix left and right.
no need or want to do that with a mono cart.