I'd have to think it wouldn't be a good idea for digital servo's as they run quite a bit warmer.
I've done it with
TRX2055 (crap servo's),
hitec 645MG's, 625MG's and futaba S3305's.
Plasti-dip or not, water gets into them if they are mounted so the output shaft faces up. Granted, it doesn't get in as easily if you put an o-ring under the servo horn, but it still gets in. I have a few fried servo's to prove it. They just got too wet bashing in the snow.
I haven't burned up a servo in the revo yet though. I've had the stock ones crap out, but the 645MG on steering and S3305 for throttle/brake are over a year old and I ran it all last winter in snow, slush, puddles... snow.
The revo has an advantage with the servo in a box. The snow or water doesn't rest on the top of the servo as easily. If it does, it's not nearly as much as it did on my maxx and savage.
My steering servo's that face down always last a long time. Of course, the one to fry and go to full rotation (kind of like butter side down when you drop toast), is always the throttle servo that faces up.
This is how my servo's look after the dip:
Here you can see the o-ring more clearly:
Like I said though, they aren't water proof, just water resistant.
I found that the hitec 985MG comes with o-rings installed, so I didn't dip it.