I'm not sure whether maxing it over Smash Mastery will give more damage since I always consume stacks against bosses with cross cut. Rarely will I get Lv 3 Blade Heart, and if I do, it'll be gone within the next few seconds. It might be more useful in parties since bosses do not aggro on Vella that easily and she'll be able to maintain Blade Heart stacks easier.
"With kindness comes naivete. Courage becomes foolhardiness. And dedication has no reward..."