If you are planting wam weather crops - tomatoes, beans, peppers, cucurbits, corn, eggplant, etc. then you can wait for spring to till at an appropriate time.
If you plant early crops - lettuce, broccoli, cauliflower, radish, beets, carrots, spinach, cabbage, peas, onions, etc., then it's better to till in the fall to be ready in the early spring.
In the midwest at least, spring weather doesn't always cooperate, and one often ends up having to wait too long into the spring to be able to till without destroying soil structure. You should never be tilling cold, wet soil.
As stone says, aerating (tilling) the soil does prompt a proliferation of soil micro organisms, and you could get a temporary boost of available soil nutrients for your garden plants. In my opinion, though, unless you have a sandy soil, better soil structure trumps nutrients every time. One can always add nutrients in a vast array of forms, but fixing soil structure isn't so easy.
