It depends. If you are on your first toon, then I would make sure and do all quests for the planet, including the heroics.
You want to do this because you'll want to gathering resourcing for crafting, find all lore and datacrons. Your first toon will likely be a major resource for your next toons that follow, so you'll need to grind it out a little.
Once you have your "supply" toon, all my next toons level extremely fast. My second toon was a smuggler / healer. I only did the main line story quests, and supplemented the other XP through warzones. I upgraded my gear to all orange, and used my "supply" toon to provide blue level gear.
My third toon was a jedi shadow, but I made the mistake of ignoring pvp warzones and opting for flashpoints and main story line only. I leveled fast, but found that jedi shadows and better for pvp than group flashpoints.
Hope this helped.