It's a mess.
It always add a characteristic, but it depends on the situation. Per to hit, Qik to fast-cast, and for the quality, it's either Intelligence or Perception. Callen and I had a small discussion about it once, on these boards IIRC, and it seems that there's some consistency to it, but I can't recal what, if any.
There:
That'd be Int for Creo, Per for finesse, mostly. Some of the time. If the stars are right.