The rule of thumb for any coin-hunting, is to hunt where people gathered, recreated, spend $$, camped, etc... So to simply go to row-crop fields, won't necessarily be any good. Yeah, sure, a field worker is sure to loose a coin now and then, but not as prolific as the daily foot traffic right next to habitations. So I'd say to concentrate around the homes, lawns, etc.... Unless you had some specific information that picnics or old foundations, or something was where current row crops now are.
As for depth, there is no steadfast rule. Depends on ground type, moisture, plowing, etc... I've dug seateds and gold coins so shallow, that I could hear them with my pinpointer before I even started digging (2"?) And I've dug memorial pennies 10 and 11" deep!