Generally, you should expose for the area that will have the minimum density on the film. With negs, that means expose for the shadows, and with chromes then expose for the highlights. If you underexpose the shadows with neg film, then there will be no information on the film in the shadow areas to pull when you print. If you overexpose the highlights with transparency film, then they will have no detail that could be reclaimed later on, if you are printing, scanning or duping, and ideally one wants the original transparency to be exposed correctly as a reference for any reproduction process.
Exposing for a middle tone or using incident metering works only when the brightness range of the scene does not exceed the latitude of the film. In the studio, incident metering is commonly used, because contrast can be controlled by controlling the light, keeping the brightness range of the scene within the latitude of the film. Many landscapists prefer spot metering, so that the brightness range of the scene can be known and development adjusted accordingly.