I till if I'm putting in an autumn planting.
If I'm not planting, there's no reason to wake up all those micro organisms and expose the soil to erosion.
Each time we aerate the soil, all those aerobic organisms get busy... this actually breaks down valuable nutrients, leaving the soil less valuable to future plantings.
An exception... When I'm breaking up some sticky clay for the first time, leaving it to winter's mercies can be helpful in breaking the clods down a bit.
