A safe backstop is something that you know (as much as is humanly possible as odd things can happen with the best of intentions) will stop the bullet you are firing.
Tim is right though, there is always a risk when you send lead across the landscape at silly speeds. The trick is to control as much as you can all of the time. Do not shoot in to a hedge for example even if you know K2 is behind it. The mountain might stop the bullet but only after it has gone through the drunk moron who decided to have a kip behind a hedge.
Shooting in to an incline can seem like a good idea and depending on the incline, could well be a sound decision. But it also depends at what height you are firing from. If you are below the level of where the incline starts, be honest with yourself and think how shallow the angle actually is from the muzzle of your rifle to point at which you intend it to impact the earth. What seems like a safe incline could actually act as a ramp for a nice little ricochet.
Ground condition is important too. Is it the middle of a mild winter with nice soggy ground that will swallow a bullet or is it late summer after 4 weeks drought and baked hard ground. I am not saying the ground will win the battle with a bullet but a piece of lead is more likely to bounce of something hard than bore in to it. That doesn't mean it cannot ricochet off wet ground.
Real safe shots are shooting in a valley from an elevated position in to a far bank. Or shooting a fox under a high seat ha ha.
I am obviously playing devils advocate with you but it is worth thinking about plenty. The more you shoot, the more you consider the consequences as everyone who has shot a good deal has had bullets behave in a way that were outside of their expectations.
Our land is very very hilly, so plenty of safe shots but actually, there are tons of unsafe shots. One particular example sticks out (normally because there is a tidy munty or raggy crow sat there) where it is what many would consider a safe shot in to a hill that inclines but directly behind the shot is a village about 600yds away. I would shoot a frangible HMR bullet into that hill but I can never bring myself to let a heavy CF round go. I bet you could fire 999 safe shots out of a 1000 at that incline but I know it is not steep enough to be absolutely safe. I have mates who would look at it and say "bollocks, that is completely fine" They are not the ones pulling the trigger though. If I was in a high seat or elevated, I would take it.
If in doubt, don't shoot. You cannot stop it once its gone.
The other thing to consider is your actually position you shoot from. Shooting off sticks does increase that angle of attack whereas shooting off the ground from a pack or bipod puts you lower and shallows out the angle of the bullet. They are small margins but can make all the difference.
