I never find cleaning plaster off my trowel to be a problem, but browning, bonding and plasterboard adhesive can be a bit of a pain.
but if you do happen to have plaster or any of the other things I just mentioned set on your trowel, soaking in hot water will get rid of it.
if all else fails, I do something I witnessed brickies doing when I first started laboring at 16.
they would use a small flat triangle shaped piece of stone that had been worn smooth on all sides and edges, dip their trowel in a bucket of water, then stick it in sand and use the stone to rub over the whole trowel. I have never had to do so as I get a bit obsessed with cleaning my trowels, but they swore by it and it works a treat.