i normally add reverb quite early on in my mixing as i see it as one of the three fundamentals to mixing - balance, pan, depth. in theory, if every part is recorded perfectly for the track so that everything fits without the need for extra EQ, compression, etc then you should be able to get a great sounding mix by just balancing the tracks appropriately, panning tracks (unless you're aiming for a mono mix) and then adding reverb/delay to create a sense of depth. normally, at this stage, you should have a pretty solid rough mix.
like many, i would say the important thing here is to not over think the reverb process and try and find the "right" or most appropriate reverb for the track. there are plenty of articles around the net looking at different types of reverbs and how/when to use them to get the most out of them. with modern convolution reverbs the choice is sooooooo vast that you can waste weeks playing around with all the different reverbs before settling for the one you like (believe me, i have!) but if you at least have a vague idea of the difference between a small plate and large hall and how it will sound it makes the choosing process much easier. sometimes, when i find myself getting too bogged down with finding the "right" reverb from the 100's in Altiverb i resort to using my old Alesis Microverb I to intentionally limit my choices. if i can't get anywhere close with the microverb i know it's time to take a break from the track and go and have a cup of tea
in terms of adding reverb whilst tracking, it
can be risky but if you do want to record with extra reverb just think it about it as being part of the sound of the instrument. for example, as heatmiser said, recording electric guitars with the amp reverb on is sometimes part of the sound of the instrument and sometimes turning the reverb off when tracking can have a huge effect on the playing and performance as player will expect to hear the reverb. Drums in a good room will sound big and sweet and using room mic's to intentionally
capture this sound is common practice.
well, that's my 2 cents
