[LDiCesare]

Fire closest. But even with that they'll often fire only once, or not all of them, because they tend to be engaged in melee before they can fire a second round of javelins.
With Pyhtium, I like to put Velites on fire closest on the side while the main legionaires stand in the middle with fire or attack closest. That way, velites do throw their javelins without getting caught in melee, if they can flank the opponent a little.