As in almost all hunting situations, the big question is—when should your rifle be fully loaded? And when should you have shells in the magazine? And when should it be completely empty?
Hey, as in all things, listen to your professional hunter. Most of the time, when you're walking, you're going to have cartridges in the magazine. But you're not going to load the chamber until your professional hunter suggests that it's time.
And hey, you know what? They're focused on tracking and stalking and looking for game. And they may even forget. So, it's OK to ask now and again, "Hey, is it time for me to chamber a round?" There's no shame in that. I do it all the time.
Lots of times, you're going to be carrying your rifle slung over your shoulder. And hey, my lifelong belief is that when the rifle is slung over your shoulder, you shouldn't have a round in the chamber. You should only have a cartridge in the chamber when the muzzle is in full control.
That means you have both hands on the rifle. You're able to absolutely control the direction of the muzzle. If you're in country where dangerous game is present, your PH is almost always going to carry a rifle. And his rifle is probably going to be fully loaded. But you're walking behind him, and so yours really doesn't need to be.
A lot of professional hunters don't use rifle slings for that very reason—because their rifle is always loaded. It's always in their hands, and it's always ready. Of course, again, in your case, that's not absolutely necessary. So it's perfectly OK to use a rifle sling, carry your rifle slung, but probably keep a round out of the chamber until you're getting very close to action.
I don't trust mechanical safeties. To be honest, I've never had one fail. But a mechanical safety is not a substitute for safe gun handling. I view the mechanical safety as a backup for those momentary lapses when you slip or you stumble, and for just a second or two, that muzzle gets out of control.
Obviously, you want to keep that mechanical safety engaged at all times, especially in the final moments of a stalk, when the rifle is going to be fully loaded. You're going to have a round in the chamber. But don't rely on the mechanical safety.