I live in the North where the snow is deep and the winters can be long. I generally only feed later in the winter once I see they are getting stressed (emergency feed).
I feed with alfalfa and once you start you have to continue to feed till they leave the area in the spring. I guess from what our local DNR states is that once you start feeding deer their digestive system changes and they cannot switch back to natural food (wood fiber).
I guess another issue about feeding early in the season is you prevent the deer from migrating to their yarding areas (Northern thing). If you feed them in the early part of the winter and the snow gets deep. If you stop feeding it may be too late for them to get to the yard which is essential for their winter survival.
So basically if you start feeding, you have to feed all winter.