Every commute is different, every rider is different, the golden rule is to use what works for you.

Simplest is a single speed. A guy here where I work uses a Trek (Portland I think, with the chain, not the belt drive) and he mainly uses his middle chainring and for one hill the small chainring, but he hits all ten on back, and said he could take off his big chainring and not miss it at all.

Pick the steepest or biggest hill on your route, find a gear that works for that, let that be your base line low gear, everything above that is just gravy and how efficient you want to be.