Seth Clifford shares an interesting approach to managing tasks with OmniFocus. Rather than the traditional contexts such as "phone", "Mac", "iPad", or "Errands", he uses time.

First, I created three new contexts: “9am-5pm”, “5pm-9am”, and “Weekend”.

Not a bad idea. As one who uses only 3 main contexts (Errands, Home, and Mac), I concur with Seth's opinion here:

Location is a modifier on time.

It's the time of the day that dictates what I will be working on, not my location. This is part of the beauty of OmniFocus; it's so incredibly flexible!