That actually hurts you more than it helps. You end up clearing quests and moving to the next zone too soon. Eventually you will reach a point you can't beat a boss and have to back track to pick up the bonus series on a previous planet. Been there.
At the very least, always do the bonus of each quest. Those bonuses really add up. Especially once you reach 47 and can start doing dailies.
That depends on if you do flashpoints or PVP. I've been as many as 6 levels ahead of my "current" planet (based on class quests) on a couple characters.