None, Roman commanders were expected to lead from the front. It was their bodyguard's job to make sure they didn't die. Every rank up from a decurion was more free to move about the battlefield, though. Commanders would thus often choose to go to places were the fighting would be most decisive. The standard place for a general would be the left right wing, commanding the reserves which are supposed to deliver the pincer manouver after the lines have joined, but still in a position to respond flexibly to developments and relay commands. The only ranks definitely kept out of the fighting are the standard-bearers (and presumably also the signallers), who would signify the general location slightly behind the battle-line for the individual cohorts. But generally, a centurion's plumed helmet is so ornate because it is meant to be recognised from behind.

The theory for this can be found in Vegetius' army manual, and some practical occasions are in Caesar's Gallic War.

By rank I take it you mean, front rank, second rank, third rank etc., and not legate, centurion etc.?