Mark, I think that is a tactic that is overlooked by most, although that statement is just a guess by me. Like turkeybuster, one of the birds I killed last year was when I decided the brush was too thick down low to sit down, so I stood behind a big pine tree, leaned into it to try to look like part of the trunk, and rested my shotgun on a protruding knot at about the right level. The gobbler I was calling came up a hill to me and didn't suspect a thing as I layed the hammer down on him. If I had been sitting, it is questionable if I would have killed that bird.
In fact, later in the season, I was contemplating using the same tactic in another similar situation, but didn't. As it turned out, I did not get a bird because I was too low. Had I been standing, the outcome may have ended up differently.
Having said that, it is a lot easier to remain inconspicuous and motionless by sitting, especially if it is going to be for a lengthy period of time.