Generally you want to expose to whats important to the scene.
The rules of proper exposure are very much so based off of a desired result (the rule I've followed is that so long as something in the scene is properly exposed - it is a properly exposed shot), You may want to underexpose a background element or use a cookie to give a pattern of underexposed areas (shadows, bars, leaves, etc). For example,
-Skyfall, Dir. Sam Mendes, Cinematography by Roger Deakins (this was Nominated for Best Cinematography)
This image does have areas of Underexposure and blown out highlights from the window - however the small portion of Bond's Face that is in focus is properly exposed - hence the image is properly exposed.
Personally, I try to slightly overexpose the highlights @ 800 ASA (it's easier to take light out of a shot in post than it is to add more light in). 800ASA from my experience is at the point where its sensitive enough for controlled lighting situations without too much grain/noise.
The best way to judge exposure is through a light meter honestly (as monitors & LCDs may not be correctly calibrated) - however if you are not in a situation where a light meter is available it's best to still slightly overexpose.