It happens only very seldomly that reverb is being added at mastering. At least, I did that only 3 or 4 times when I restored old vinyls.
Just to add some more depth and a little stereo room to the mono sound.
I found it a better way to leave it to the last stage to decide what was best for keeping the most original, yet, most attractive sounding reproduction.
I have never done that for contemporary productions, though. But this is not a total NONO... whatever needs to be done will be done...

I can't think of any reason to wait for adding reverberation till the production lands on a mastering engineer's desk.
For amateur recordings, were the recording is already "the mix", I could imaging to add a little this and that...

The Wavelab manual mentions doing this to add "sheen" or "polish" to the mix (master). I've never felt I needed to do that, but there ya go. THe pros at Wavelab consider it a possibility.

Perhaps roll-off the low end on the 'verb to avoid mud?

It's really going to be content dependent, and what kind of verb you are looking to add. Somethings lend themselves better to short small verb and other things to big verbs. It's going to be tough finding a reverb that's going to play nice with a dense mix. You're much better off addressing that in the mix.

O.K. Thanks for the answers.

Can't imagine waiting -- I certainly wouldn't want someone putting reverb on top of a mix when I want a certain amount on the snare and a certain amount on the vocal and a certain amount on the toms and (etc., etc., etc.) and probably none on the kick and none on the bass and (etc., etc., etc.).

Those are mixing decisions.

I've used reverb plenty of times to mask something - Or 'repair' a bad fade or what not -- I've automated reverb sends to go over fades to add 'distance' to the reduction in volume - But I'd generally not want to put reverb on an entire mix unless there was a very specific reason to do so.

Spot on !!