(Apart from fixies, which I don't really know about) chain tension has surprisingly little to do with keeping the chain from slipping when pedalling, so the main reason for controlling the slack is to avoid accidental derailing due to bumps and jars.
OTOH you don't start to see excess friction until the chain goes really tight, so there's little point in running it slack deliberately either. A half-inch sag or thereabouts is the happy compromise.