afraid or not is generally a role play thing, much like angry or not is a role play thing. . .I think the RR situation is where it's a check to see if you panic.
The GM can decide to make "Panic checks" whenever they choose works in their game, but if the GM designates any singular thing as "supernaturally fear inducing" then it's another matter.
In one game-world I have run, undead are unlife, and the energy field/aura around undead pokes the fear/panic button in your brain directly. . ..no matter how often you're exposed, they always do it. . .even if you can't see them, or are behind a barrier like a wall, when you enter a given radius you start to feel them. . .expressed as an RR vs Panic. . .you can't really learn to get over it, it just always works that way. . .you are always afraid, the check is to see if you're afraid and still rational, or afraid and panic. You can go up levels (or spend more on RR will) making it easier to not panic, but you can't reach a point where you can skip the roll, because it's an involuntary, irrational effect, much like you can't choose not to breathe.
IMO that's what the check is, it's a panic check. . .and when to use them will vary from game to game. . . .
I must admit, that over the years, I've had more success with the "Make the players paranoid" method described by Mitchiban than I have had in just saying "You're scared." "Now roll for panic" . . .though at times you need to go that route, especially if you fail, and the players choose not to get into the scaridy-cat groove.