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.
A lot of folks don't get into PvP, and flashpoints can be hard to get if you are DPS.
Healers and tanks can often get insta-ques, or very short waits. (for this very reason, my alts are trending heavily towards healers)
But even in flash points, it's better to do the bonus when leveling. In my experience, groups that try to skip mobs are substantially more likely to wipe. Many fights can be skipped easily, but some groups are so close together the margin is barely wide enough to walk through, no room for error. Especially for non-stealth.