If I'm away from an airfield and my engine shows signs of misbehaving, the first thing I'll do is climb, the second is fly to the overhead of the nearest alternate. I'll descend only when I can stay in gliding distance of a runway. If the engine stops en-route, then the extra height maximises my options.
Hear hear! (Or maybe that's here here - I've never been sure). Been there, done exactly that, twice. Once was when (as it turned out) my turbo had been busy reducing its impeller to shards of alumin[i]um, the other a never-explained engine hiccup. In both cases I circled down over the field.
In a plane there's very little the engine can do that is immediately life threatening - falling off, catching fire, or shedding the prop, maybe. Just running a little rough or oddly isn't necessarily a hint that one of those is about to happen, especially if the issue doesn't recur (as it didn't in my two cases). Anything which suggests that any of those catastrophic events might occur is a reason to treat it as though it has stopped and get on the ground asap.
The heli is a different story. Among all those bits whirling around are several which can make the whirling stop. A "chip light" indicating metallic debris in one of the gearboxes is a reason to get on the ground RIGHT NOW.