I think there is a certain amount of "when in Rome . . ."
I have never used the shoulder carry in North America, and I think if I did, someone would tell me not to be a jackass. On the other hand, as has been pointed out, it is used regularly in Africa, by PH's as well as by trackers who may happen to be carrying a rifle. I've done it, and while I make very effort to point the muzzle to the side (I am never the first in the line), I can't swear that I've never covered someone with the muzzle. Having said that, I believe I have a more developed sense of safety than most, if not all, of the trackers I have seen carry weapons.
The presence or absence of a sling may make the gun carrier more or less likely to use the African carry, but that's for convenience. It's got nothing to do with safety.
My conclusion has been that if the PH is ok with it, and you keep safety foremost in mind, then you should feel free to use the shoulder carry. After all, many hunters do, and those in a hunting party who might have been shot by an AD don't seem to have been shot while a rifle was being carried this way, at least during the years I've been paying attention to these things. So if experience is a guide, perhaps it's not so unsafe after all?