Really, "Don't roll if there isn't going to be an interesting failure" is the biggest rule I follow. Hell, my players only have to roll out any thaumaturgy spell under 5 or 6 shifts when they're in the middle of a fight, because the worst thing a failed roll is going to do is give them a mild consequence, which will only last through the next scene.
Though here's an interesting alternative, finn: Compel them with a "succeed, but..."
So if that wallflower goes for the awesome but doesn't have the stats to pull it off, compel their wallflowerness to say, "Okay, you shove the innocent bystander out of the way, but you're too unathletic to escape the blow yourself, so that's a moderate consequence," or "...but now you've got the ghoul's attention on you."