If you've mapped/sampled your field(s) with a GPS, I reckon it would be lots easier to apply water/fertilizer via the pivot against the yield/soil maps in GPS form rather than degrees of rotation.

As for controlling the corner units: it would be the only way to go. All the guys I've talked to in Idaho who have/had corner units with the buried wire described the buried wire guidance system as a disaster waiting for a lightening strike.

For linear irrigation units, I think GPS would be the only way to go. The furrow-follower units we used to have were a really bad joke.