Is it necessary to bake my mushroom caps before stuffing them? And if so, for how long and at what temp? I want to make the mushrooms the nig

I brown the bottoms in butter first

No never. Stuff them then bake them so they suck up the flavor. Depending what your stuffiing with 350 - 375 is a good idea. Until done 20 - 30 minutes.

I take button mushrooms, scraping the under underneath the cap out for more stuffing room, brush the outsides with olive oil , then stuff. I place them in the oven at 350 for 15 to 20 minutes... Or until stuffing is hot. Great every time .