Barring some sort of pressure of time (or resources, or otherwise), the optimum route to clear out the dungeon is to be nice and systematic. So always go left.
(Of course, always go right would work just as well, as long as whatever is chosen is done consistently. But I've always done "go left", so... Oh, and @Seramus
is right about the handedness issue.)
And of course, as a DM, it is of course incumbent on me to make sure that such systematic approaches aren't optimal - either because of traps designed to move the party against their will, invisible teleporters or other weirdness, or simply by applying one of those pressures I mentioned at the outset.