I always wear a harness in any treestand. I use the four point harness that was supplied with the Summit stands. It is easy to put on and I never have an issue with noise. It actually helps when taking a shot in bow season, I set the tether just long enough where I stand on the end of the stand and feel it tighten up, so when I am working a shot it helps me know where I am on the stand.
The buckles can make noise if they hit the side of the stand or something. Many times I will put my harness on at the truck and were a coverall over it, end of problem.
About the ground blinds, my wife has had a doe actually stick her head in the window so I don't think they spook deer. that same morning she had a monster buck 40 yards away and was shaking so hard she couldn't hold the scope on the deer to take a shot. She's getting better though she actually pulled the trigger last year, didn't cut a hair on the deer, but at least she took the shot.