FX,
I dont think it's "cheating" persay. It does give you an advantage but only a slight one. Usually, by the time you hear the guy attacking you have maybe 4 seconds to find out where he is and how close. If your not jinking about, which you should be, then more than likely your hosed or at least gonna get banged up. So the advantage is SLIGHT. If you have propper SA, which you do, you'll see the guy Waaayy before you hear him. As far as GV's go, well most people turn the engines off anyways so whats the point.
Is it "ethical" to do so? Any pilot that enters a fray on equal terms isnt flying right. Whether it's coming in with Altitude or Speed, both are still an advantage. So Ethics just dont fit in the situation. You fly to fight and you fight to win.