I’ve never played either, so I can’t comment there. But while I’m an Alyson Hannigan fan–she’s much better looking that Gellar–I don’t think Willow is all sweetness & light. She was evil a long time before going all Dark Phoenix, and even before resurrecting Buffy. She was just evil in a sneaky way that happened to let her be allies with the genuinely heroic (however bitchy) Buffy Summers.
But we get exactly that deep-end sense at the beginning of Tabula Rasa. She feigns not being ready to go – standing there in a towel – to get everyone else to leave the house. Two seconds after the door closes, there’s a magical sfx and she’s dressed and ready, and retrieving her implements to cast the forgetting spell again, notwithstanding her seemingly heartfelt commitment to go a week without doing any magic. In fact, that whole promise arises after Tara observes that while Willow’s use of magic may have been altruistic in the past, now she’s just “fixing things” to her liking.
And sure enough, the spell she casts is intended to make things easy for her, by making Buffy forget being pulled out of heaven and Tara (again) forget her ire at Willow. There’s no agonizing over a moral dilemma – there’s just quick, efficient spell-casting.
That’s not someone who’s doing it for the second time, especially after the first time created such a harsh and betrayed reaction from her lover.
I picked often, because of her ease and comfort using the spell exhibited in the episodes in question. It’s unclear whether this really suggests she was using it often on Tara, or the Scoobies in general, or on various bad guys or random innocents. But she did have a pattern of using magic whenever she could to tidy up little discomforts or inconsistencies in her vision for her world. It’s not out of the realm of possibility that she may have have used the spell on Buffy, Xander, or even Giles to smooth over some little spat or what have you.
So I speculate she used the spell on Tara more than three times, and closer to the realm of “a lot.”