Tom,
Most outside contractors have a built in factor for reasonable changes, expecting to rough something in and then change it to some degree when the architect (head guy or associate) shows up. They can get their dander up if every green needs changing or a few greens need repeated changes that don't progress forward towards completion. Or, if the change requires new drainage or comes after once approved and irrigation is installed. I will make changes on the first visit that are pretty major, smaller on the second and really minor tweaks on the third viewing of a green. I call it the "yards, feet, inches" syndrome and do try to get most right before hand, even if it means roughing in a sketch after I see the clearing or first shaping so they at least get it close the second time.
I was thinking most of the Fazio stories I have heard, where they raise or lower a fairway a couple of inches for artistic reasons. Unless that is the critical elevation for vision, drainage or flood protection, I can't recall having changed a fairway like that later. I do keep in mind that if I am the only one who would really know the difference, i.e., doesn't substantially improve playability, maintenance or aesthetics, and it slows the schedule, I let it go.
Somehow, those late changes cost the Owner something, and one of us architects will eventually be taken to court for the lost revenue from opening a year late (along with the contractor, engineer, etc.)
I have always been of the mind that they really, really, really better be worth it. There is probably a discussion to be had on just how to measure improvements made later in the design process. They are necessary, and I doubt there is a course I have designed where I wouldn't make changes years after the fact. So I do try to get it right through numerous field visits (me, not associates) at the right times. Like you, my fee is fixed so all that cost my owner is a few more air fares than normal.
That said, there are definitely excessive field changes no matter how you look at it. And, about a thousand stories about nearly every signature architect of how unnecessary those changes were. Another signature designer had it in his contract he could send in the shapers after grassing for some final tweaks. The shaper spent two weeks shaping little mounds around tee complexes to spruce them up. In no way did those improve anyone's golf experience, but the architect had the power to do them, and so he did them.