I do it a lot--helps concentration. People are seemingly always amazed that I can fiddle in the dark......well, there aren't any frets to look at anyway. I have to listen to see if I am OK on intonation, anyway. I do try to avoid in in front of an audience, tho. Sessions, practice, etc. it's OK to just close your eyes and get lost in the mix, as long as you don't lose your place. I do that, sometimes too, LOL!