That freaked feeling is healthy. Never ignore it, but try to learn (gradually) what is safe and what isn't. As others have said, there are many variables - soil softness after a wet spell, woodchuck activity, etc that make any hard rules about what angle is safe meaningless or dangerous. Most important advice I have is to pay attention - don't get distracted by anything that doesn't have to do with your stability when you're on a slope. No cell calls, no waiving at the neighbor's pretty daughter ... you get the idea.
Know what to do if you start tipping - turn down the hill, drop the FEL & 3PT, etc.
1) If you have your ROPS up, you should have you belt buckled.
2) If your ROPS is down, you should be unbuckled and ready to bail to the high side.
These two states are mutually exclusive - you won't have time to buckel or unbuckle your belt to match you ROPS state when you need to, so make sure you have the right combination before you start to move.