Without specifying a rank, you're get the highest rank you know.

Blizz has nerfed down-ranking our spells, so you are charged the mana for the highest rank you know, even if you cast a lower rank spell. So aside from landing buffs on toons too low to be buffed by your highest rank (the downranking of buffs happens automatically), I cannot think of a reason why you'd want a lower rank.

The only way to really downrank now, is to not train higher ranks. If you have enough +healing, it might be more mana efficient to use the lower rank.

But since you are always charged as much mana as the highest rank you have, and not specifying a rank gets you the highest rank, I would not recommend even bothering with (rank2) or whatever.