It depends on the fight. Some fights it's easy to throw out a HF or Smite once in a while, then consume your stacks (even if you don't have 5) to get a nice throughput cooldown. Others, you may prefer the Strength of Soul and Train of Thought effects, which give you some extra time with Inner Focus when using Greater Heal (very nice for tank healing) and more frequent access to PW:S again when tank healing.
Keep in mind that the second half of Heroic Halfus aside, there is not a single raid encounter where Smite will be a core heal. Holy Fire does *okay* HPS, but Smite is merely used in periods of low damage, or simply weave one in once in a while to get your stacks up for a predictable damage spike (good example is Chimaeron, try to weave in a few HF/Smite to get a few stacks, then for the Feud you would pop wings and spam Prayer of Healing).
Any way you cut it though, leveling and gearing up in 5-mans... Smite will make your life significantly easier. There isn't as huge of a difference between the two subspecs as many people let on, you should have no issue going from one to another at 85.