Ladies and Gentlemen,
The answer is...... Don't do it. Simple, it causes unwanted stress in the transmission and engine.
One other point the is possibly even more important, is that you do not want to put a lot of stress on your engine at a low rpm. This would be, chugging up a hill in a high gear, or pulling a large load up a hill in a high gear (low rpm). If you "bog" the engine down over time, I have heard you can bend rods (not sure if someone else can confirm this), over heat the engine, as well as over-tax your lubrication system? This of course is far from ever occuring at a stop light while on a hill, but just one more thing to consider for engine and transmission longevity.