I'm not an expert in technique but I think 8 should be used only in perfect light conditions. Personally I don't give priority to aperture, I prefer to give priority to shutter speed and keep it at least higher than 1/250.
Once I made 1500Km to Madrid by car, only to photograph planes. There I gave priority to aperture keeping it all the time at 8. I caught bad weather, cloudy days, the result were lots of blurried photos so I learn the lesson in the worst way.
Now I use all the time high shutter speeds, when the light doesn't permit me to use high speeds it's time to go home and be a "bad" guy at the Airliners.net photography forum