It depends. Usually, if a pokémon acquires a habit in one stage of their lives, they carry it through to their other stages unless it’s corrected, yes. As such, not only might your dartrix rest on your head, shoulder, or bag, but very likely, your decidueye might as well. It may be a good idea to train your dartrix not to do this—but only after it will evolve, so it will be easier to teach.
Of course, there’s also a possibility that this behavior will only exist in your rowlet. As I’ve said, the passing of behaviors from one stage to the next is what usually happens, but in some cases, behaviors drop when a pokémon’s personality shifts. It really depends on the individual.
To be honest, though, it’s not something that should be cause for alarm, I should think. It’s best to observe your rowlet before and after he evolves, but let him evolve unhindered nonetheless. Once he gains mass with his new form, he’ll be able to recognize what he can and cannot do through trial-and-error.
The Pokédex According to Bill 