Well, I think the core threshold slowdown (nvidia utility) will remind you if it's exceeding the temp limit by flashing something on the screen (although I have yet to see this happen--but I'm guessing it will remind you somehow). I ran my PC for two weeks straight without shutting down and I never saw the temp of my GPU spike, only when I am playing games or running some applications. If it does get really hot for no apparent reason, it is failing. I would'nt bother with it anymore. Especially with all the headache of worrying that the GPU has heating problems (which I don't think you do-- a bit on the high side, but nothing to worry about. It's well within tolerable range).
If it starts to malfunction, your screen will freeze, perhaps even smell something burning coming from within the PC, most likely you will get no picture either. You may see error message pop up in regards to your video card. When that happens, check the temp again to see where it's at and go from there. Once the card overheats for a period of time, it will eventually fry the circuitry.