If you're chasing a perfect sear, mushrooms are a huge pain to deal with. Whether or not you rinse them, slicing and chopping alone releases enough of their natural moisture that even previously bone-dry mushrooms can turn soggy before they hit the pan - and that simply won't do.
We all know moisture is the enemy of crispy, golden-brown goodness, so keeping mushrooms as dry as possible is key to a successful sauté. There are two prep tricks I use to minimise moisture, both of which I learned from this Bon Appétit video on seared mushrooms.
First, I wipe 'em off with a towel rather than rinsing. Rinsed mushrooms don't actually absorb all that much water, but interior moisture isn't the problem: even the quickest rinse renders the surfaces slimy, damp, and impervious to towel-drying.
Second, and more importantly, I de-stem and tear mushrooms up with my fingers rather than cutting them up with a knife. (Don't throw those stems away; they make excellent vegetable stock.) This breaks the mushrooms into smaller pieces without destroying the fibrous structures that hold in water, which ensures that the exposed surfaces are nice and dry when they make contact with hot fat. It also takes roughly half the time.
The tearing method won't work for all preparations — it's probably not a great choice for duxelles - but if you want to showcase the natural texture and flavour of fresh mushrooms, it can't be beat.
Carla Sears Mushrooms to Crispy Golden Perfection [Bon Appétit]