In my mileage-wise 8 trips around the planet I've never attempted to seal valve holes. Granted, always with aluminum rims. Nearly all of those miles were in or around Boston, Ann Arbor, The Bay Area, Seattle and Portland; riding year 'round. Rainfall 38-42" per years in all those cities. Some of my rides have been epic wet. I've ridden through water almost up to the BB. Rain that had the frogs cowering under whatever shelter they could find. I'd a thunk the issue of water in my rims would have raised its ugly head by now - if it mattered.

Oh, I've never done anything to dry my bikes after those ride. Just park them leaning against the shop table. Later or next day I hang them up. (Dirty or salty rides, they get hosed off before coming inside - if the hoses are hooked up and the water on - think freezing winters.)

Now granted, I've never ridden tubeless so there might be issues there I don't know about. My miles are split between clincher and tubular. The deep rides were on clinchers, the frog terrors on tubulars. My pedals have gone underwater on both.

And edit: My current tires are Vittoria tubulars with smooth valve stems. These tires are clearly made to be ridden in all weather yet Vittoria seems to have zero concern about sealing the stem. Their target audience doesn't do aluminum so that suggests that Vittoria is OK with us doing nothing to seal those valve stems.