If you're sub, you'll stay at the right level with just your class missions. If you're F2P/preferred then the planetary storylines will supplement the missing XP (and maybe the odd flashpoint).
But realistically it doesn't matter too much since you scale to the level of the planet you're on. You're still pretty danged OP if you scale down so it's not gonna be much of a challenge, but I don't think anyone is in the leveling experience for the challenge : P