I rarely do a maintenance and find nothing wrong, some of the newer installs that came from new construction I tend to find less, but older equipment and ductwork I generally run into problems. but with that being said I go through the whole system, in the attic, under the house I dont care, ive found more ductwork problems from rodents and duct tape then I can believe. then theres a very thorough work sheet I go through, from amps, electrical, disconnect, capacitors, pressures, temps, subcooling/superheat, static pressure, and many other tests that my equipment does simply by poking a couple holes in your ducts/plenums, but to be honest a good trained eye will catch and look at ten times whats on that paper. But there is more, or should be more involved in a maintenance plan, like priority calling, atleast 10% or more discount on parts and labor, a warranty on parts the company replaces on your system, discounts on upgrades you might decide to make, things of that nature.