I've always gone the "all or nothing" route. I know it's not the most "efficient" way to level in this game, but more than anything in this game I've always loved the bond or identity that I feel with my toon. That being said I always gravitated towards healing, in Nos I leveled both a Resto Druid and a Holy Priest from start to finish. I always found that juggling specs and gear made my character and experience feel more shallow, I understand this is personal opinion. There is more than one way to play this game, there is nothing wrong with your journey being strictly tank or heals all the way through. I think it shows dedication to the role that even when it's an uphill battle you are still sticking to it.
I'm debating on Lock, Druid and Priest, but if I end up with the latter two I will be going heal spec from level 1-60.