Well, since this is an important point for many of us, I just looked up every reference to
athelas in the
LOTR .
When Aragorn use the leaves to attempt to heal Frodo in Rhudaur, Rivendell, & just after Moria & before Lothórien, he only crushes or boils them - he never says anything per se, certainly not a spell.
His efforts at the Houses of Healing after the Battle of the Pelennor Fields, however, are different. Here's what he does to heal Faramir:
Then taking two leaves, he laid them on his hands and breathed on them, and then he crushed them, and straightaway a living freshness filled the room, as if the air itself awoke and tingled, sparkling with joy. And then he cast the leaves into the bowls of steaming water that were brought to him, and at once all hearts were lightened.
So, I guess you can say the whole "breathed" thing means some type of power passes through him, though he never actually utters a spell.
When he heals Éowyn:
But he bent and kissed her on the brow, and called her softly, saying:
'Éowyn, Éomund's daughter, awake! For your enemy has passed away!'
He then:
...bruised two leaves of
athelas and cast them into steaming water; and he laved her brown with it, and her right arm lying cold and nerveless on the coverlet.
It gets trickier, because he 'calls' her as already described & then again & leaves the room, commanding Éomer to also call her. He does, & she awakens. Thus, if Aragorn has some forgotten power of Westernesse, as the text somewhat implies, then he may be casting a spell when he calls her, but how do we explain that Éomer does as well? It seems to me that it is because he is her brother & does so from love, rather than any magical ability - which Éomer otherwise never displays. Therefore, my two pence is that Aragorn does not cast spells per se, but uses his healing skills with
athelas.
In game terms, this is probably best played by providing an appropriate numerical bonus or success dice to
Healing for someone of the blood &/or using
athelas, say, plus 1 or 2 (or whatever works for your game).