Which traces should I fill / pour?
Assuming two layer board the most common practice is to pour the ground on both sides.

There are three reasons i can think of using a pour for other then a general ground. First, in a multi layer board (4 or more layers) one inside layer is a power pour and the other is a ground pour. Second, when running very large power traces it's easier to run a regular skinny trace and then use a pour to make it larger. Finally a pour can be used to heat sink components like D-Pack MOSFETs (turn off thermals in that case) and voltage regulators. In the case of regulators the tab is on the ground plane but similar to the MOSFET thermals on just the regulator need turned off.