In reply to J Whittaker:
> Do most people carry pitons and the like for winter climbing and at what point do they become useful?
My default is always to carry them. They are used occasionally, but normally in the kind of situation where you'd really regret not having them. As you say, it is more about the rock type than the grade, but in my experience, they are more likely to be needed on easier routes (say III to IV), and particularly gullies.
On popular routes, they are often in situ, but otherwise, it is wise to carry a small selection of knifeblades and angles to use as a last resort.