Sitting against a big tree breaks up your pattern and also protects you from shots from other hunters (you're all camo'd up ya know).
I went out Fall turkey hunting for the first time last week. 4 day, either sex lottery seasons in MN. I got out before the season and scoped out some turkeys near a big tree. For the opener, I got out 45 minutes before sunrise and started setting up -- I thought I'd try some decoys. A female was yelping at me the entire time I set up in the dark. I tried to be quiet but I still banged around.
As the sun came up, I heard a few turkeys and saw a turkey head for a fraction of a second at about 30 yards in some tall grass. Thinking "this is easy...", I waited for her to come in closer.
Needless to say, I never had another shot. When I quieted down, the turkeys disappeared. In the Fall, they seemed to respond more to my human sounds than any turkey sounds I could make, but I wouldn't necessarily rely on this tactic.
In Minnesota, you need to use a shotgun for turkeys, so you need to be very close. I'll probably put a scope on my Mossberg 835 for my next lottery turkey hunt and dedicate it to the 2nd place Nat'l bird so I can scope the thick stuff easier.
In the Fall, take a shot when you can. Regardless, it's great practice blending into the woods and it makes you a better hunter for everything else.