Tips are not a factor on 90% or more of my jobs which put me on stages out of reach of the audiences. And I never put out a tip jar as I don't feel it is appropriate at the places I play.

However...I do have a casual lounge job where I am positioned at dance floor level. When I first started doing this job, I played while standing and no tips. Then, because of a back problem, I began sitting and people started depositing tips on my powered amp which sits to my right at keyboard level as I play.

Hard to believe, but the only thing I can pin down about all of this is the fact that I am sitting...even though it is an elevated stool. Just something about the casual nature of the job, I think.
Eddie