What about relying purely on good positioning with no extra tools used to steady the rifle? I think it sounds like a good idea to practice it. I have to admit though, that I’m always looking to use whatever is handy to gain an advantage. But sometimes all you got is what you got, and it’s good to be able to get by without extras. In my experience, however, the sling will tighten groups up considerably. It’s hard to put a number on it, but I would say that the sling could potentially double your accuracy over using nothing but good position.
Kneeling without sling, 100 yards, 6/20/11:
Kneeling with loop sling, 100 yards, 6/20/11: