One spot we used to hunt deer, we called "hell hole". It was so named by the guys pushing deer.
It was a cut over, with blackberry and raspberry bushes so thick you really used your rifle only to push ahead of you. The canes constantly robbed you of your hat, and tore your clothes to ribbons. Yet deer seemed to move through it effortlessly.
It was no place for a sling. The pushers never did get a shot when we hunted there. You couldn't see ten feet most of the time. One guy said he probably should just leave his rifle in the truck for that run. Guys on stand did well though.
I'm a rifle at the ready type guy, and that practice has accounted for a lot of close game.
On stand, yes, I'll have a sling. I also used it when we hunted the big marshes north of Sudbury where you could see pretty much a kilometer.
I have to comment on the guy who, on stand, has his rifle slung on his back. How much game has been lost in this manner? Now some of the Western guys will not understand that I expect, but, when pushing deer, you may have seconds, or, A second to make a decent shot. The deer may be standing, looking at you. But it won't be when you try to get your rifle off your shoulder.
One of our guys uses a sling that holds his rifle (he uses a P17) across his front in the ready position. It automatically comes up to support his shot when needed. I can never remember the name of it. Wonderful system for a hunter that walks.
Hunting conditions differ. When I hunted BC I used a sling all the time.