I'm with Alpha on this... I used to be one of these 'leave me to do my thing' mixers, but more recently I have found that I can get mixes completed quicker, and don't bust my own ass mentally, if I have the artist/bend member(s) present when I mix.
I do like to get a few things sorted out first though, like all the imports and routing, before the clients arrive.
I have over mixed a few projects in my time, but I have never done this with clients present... that's why I appreciate having them there and I have no insecurity whatsoever because I know I can 99/100 deliver what they want.
I'm also not in the business to be a hermit.