For blues/jazz walking, rolled all the way off, or nearly so.
Same here. My P is a '78 fretless that I use mainly for jazz. I roll off the tone about 75% and have the amp pretty flat. People say it sounds like an upright, which of course it doesn't really, but I think it's just their way of saying it's mixing well for jazz.
It took me a long time to discover that tone knob, but now that I have I realize its power!
