Reverb in the mastering process

I'd add an overall reverb in the Mixing Stage, never in Mastering.
That way, for mastering, all that's left is EQ and Compression to smooth the 2-track.
 
Maybe it's me, I can't think of a scenario where I'd want everything summed to 2 tracks and THEN uniformly add reverb.

I would say (only guessing here) that it could be used in small amounts for a "glue" effect. I would also say that it's probably done regular in certain situations.

Jeff Evans
The only time though where this may work is for example if you are mastering a classical recording say and it is very dry due to the way it was recorded. Then a little reverb can go a long way but this would only apply in situations like this perhaps.

+1  I will use reverb in the mastering stage for classical recordings in some situations.  I will always send each track as needed to a reverb buss during the balancing (mixing) stage.  But often times, in very dry recordings, too much reverb initially will sound obviously artificial.  I find it better to add a little bit during balancing, and a tiny bit later during mastering to add a bit more breath to the pauses.  
 

I am not a fan of this either, which certainly does not make it wrong, but the best way to convince yourself is to do an A/B of tracks while mastering (pre vs post). A friend sent me a track over the weekend complaining about two issues - his voice wasn't like another track he sent, and the master that had been done he didn't like. The difference in his voice was mostly gently reverb on the previous track, and the issue with the master was reverb on it. Without stems, I could only fix one of the two (the mix was actually very good from the same person that mastered it).
 
A visual to consider... if you think of a sonogram (frequency spectrum over time), reverb is essentially "smearing" that spectrum forward (imparting several renditions of the entire track on top of itself), so all the effort to avoid collisions in a mix can be undone with reverb on a master. The master he sent suffered from this, and it turned the bottom end into goo. Be very wary of what a plugin is doing and its limitations, i.e. if it processes everything the same way, on a master track it won't know vocal from guitar - it is simply "one size fits all" and does its thing.
 
Again, depending on the content and application, this is not guaranteed to destroy things, but definitely A/B the pre/post masters to convince yourself.

Read this thread:
 
http://forum.cakewalk.com/DIY-Mastering-in-SONAR-m3487978.aspx
 
I prefer not to master at the time of the mix. It is actually quite unwise.  Wait a week and make it a totally separate thing. Then you can make much more correct decisions about mastering.  For example a week later you might actually hear the reverbs you have applied in the mix and you may not want to add any in mastering at all.

Sometimes I put a gentle reverb on a master track instead using a reverb bus. Makes the impression of a live recording in a room. There might be also some spring reverb on guitars as if I used the amp built-in reverb. But that is the mix decision, not mastering.

I mix down to a 2-track master before importing that one track for mastering in a separate project.  Primarily because the LP EQ and Compressor take a lot of memory and it eases the process that way.