Well, honestly I want to play when I am stressed out. Playing my bass is the best drug for tense situations because bass playing is much more of a physical thing and by the time I am relaxed, I can stop. It like shock therapy.
When the pressure of the world ware me thin, I play myself tired until I am relaxed and forgot what was the fuss-all-about.
However, when playing with a band, that a whole different thing. I am most relaxed and for the most part, I am already in a relaxed atmosphere to begin.
Yet, there are times when I am alone, and I can crank it up a notch or two, I would be in a zone so deep, I am like on to a monk; completely relaxed and totally unstressed. But most of the time, just before I play, I am pretty stressed, and thats better than firing up a square.