In the early AM, you have to assume there will be deer feeding in these ag field. Therefore, IMO, you best option is to slip along the fence row. This will limit the number of deer you will bump and help conceal your movements and being skylighted. This would go for both when the crops or in, as well as when they have been removed.

PM hunts are little different. If you are getting to your stand before the deer are entering the ag fields to feed, I would see no reason why cutting across (after the crops have been removed) an ag field would hurt anything.