I'm at 29 with mostly the Heal tree. The DPS from Commando even with a heal spec is so great that there is little need to get much from the other trees. I use Aric and I rarely have to heal him for much, even against elites, because I work to stay geared and as many levels ahead as possible. 5 or 6 lvls ahead makes the game go very fast, and it makes the heroics doable even with poor pugs. Aric can take out Golds by himself, and together we take out Champions and bosses very quickly. This is my 4th alt into the 3rd act (with 2 50's), so I have an extra 230 points of presence and that makes a huge difference as well.
I will set 4x up, but I hope I don't have to use him. Also, too, Elara. Sometimes you just have to be on the boss so you can get the interrupts done.
Finally, get the exp boost backs from the Cartel. 25% extra exp on everything for about $1 a level. Very much worth it.
So, yes, level as a healer. I have leveled a sage and a scoundrel as heals and ti went fine - as is this commando.