You should absolutely use Sleep at low levels depending on party composition and dungeon. Look at your other DPS. Is it a melee? He probably doesn't have area of effect (AoE) enough to be worthwhile. Is he a Dragoon? Dragoons don't have AoE damage early on. Do the mobs hit hard (Brayflox dungeon)? Yeah, go ahead and sleep the extras. Do you have a Warrior tank? Wait until he stops using Overpower to sleep things. Is your tank taking too much damage (undergeared, lower leveled)? Sleep.
Do you have a Summoner with you? They'll put damage over time effects on all the targets. In that case you might as well AoE all the targets alongside him. Is your tank pulling absolutely everything around? Go ahead and AoE. Do the targets have little health and would die quickly? AoE.