Oh, I never get high before or during work (not even when I worked graveyard at a gas station), although I did used to keep a joint in the car ashtray to smoke as soon as I got off. I may be a pothead, but I DO understand that it is a drug, and just as I wouldn't drink a six pack before clocking in, I wouldn't smoke a bowl, either.
Of course, I pretty much always smoke before I do schoolwork, and I am also pretty sure I was damn stoned during at least part of the time I was drawing the logos.
So, I cannot guarantee I won't be stoned while doing any free-lance work, but it doesn't seem to effect the quality, does it? Of course, maybe if I didn't smoke before drawing, I might be headed for the title of ARTIST, rather than GRAPHIC DESIGNER, but much of the art out there makes me seriously doubt that, too.
On a side note, I am out of pot right now, and devastatingly sober.