A common goal of some Arkham Horror house rules, or so it seems from internet discussion, is making Sister Mary more of a contender in the struggle against squamous horror. In fact, that even became one of the modifiers in the difficulty level cards published with Black Goat of the Woods: Sister Mary can spend clue tokens to reroll to keep her blessing.
Personally, I think that’s a little too low key for the lady. Her stats are unimpressive enough she needs a stronger edge. So in our games, she doesn’t roll to keep her blessing, period. She can still be cursed and all that, so she doesn’t have a permanent blessing. Most of the time, however, she can expect the benefit of 4s counting as successes, which can only improve her chances when she’s maxed out a skill to a whopping three dice.