Yeah, most of the time, I end up just letting my players go ahead and get away with a spell that's 6-shifts or under. For the most part it's one declaration needed to cast, and the worst thing that's going to happen to them on a blown roll is a mild consequence.
For bigger spells (10-12 shifts thus far), I'll usually have a couple of the declarations be a gimme with some time cost, and one or two involve a big of sidequesting (in the last game, one spell involved going to the south west US to capture lightning with a bounty hunter on the way, and another had a character having to sneak into her work to talk her boss into lending her a component). I tend to make them roll out the casting for those spells as well, since there's a real chance of significant consequences if they blow it, and I started a battle right in the beginning of another casting for the tension.
But yeah, generally Thaumaturgy's supposed to be the "safe" kind of casting.