If I'm cooking beef & mushrooms w/ wine in a crock pot tomorrow is it okay to add everything to the pot and let sit in the fridge overnight?

It should be fine, but as noted be sure to brown the meat first. You might want to add a bay leaf or two also.

I would not let everything sit together overnight. The flavored will meld too much and you will get a very one note dish. Sear the beef before you sautée/sweat the veggies and put everything in the crock pot. This way you can hopefully avoid that one note dish.

it will probably be better actually, at least in the case of the meat. If you can though you really want to sear the meat, then sweat the vegetable and deglaze with the wine before letting everything cook together other wise it may turn out a bit astringent