I'm glad you asked this. I've been wondering too.
I've got myself in a bit of a tiz, because these brushes are not cheap enough to ruin willynilly. And some are such works of art, that even if they were cheap, you wouldn't want to ruin one anyway.
So I heard that its not good to get water in the knot, hence why we have stands and don't leave them on their handles to dry.
Ever since I've been scared to get the whole brush wet, and have been trying to just get the hairs wet. This is tricky.
The funny thing is that my first proper brush (ignoring my horrific supermarket £3 one) - an unbranded badger hair that came from TK Max and cost about £7, has spent the last 2+ years being chucked straight in the sink with warm to hot water that my razor goes in, my flannel goes in AND the bowl sits floating on, keeping warm.
The brushes handle is wood and after about a year I noticed the varnish getting misty. BUT its still absolutely fine, its not shedding hair any faster than I would expect (1-2 hairs per shave).
But when my New Forest brush and EJ brush arrive I wanna treat them right.
Do we bung them, handle and all, in the water or stand them up so only the bristles are getting wet?