Bob, I do a GPS track with every course I measure. I lay it down when doing my straight-through ride.
While I have checked in the past and found that the length is normally not correct, I don't even look at the length of my GPS track any more. Too inconsistent to even consider.
I do notice, though, as I map my course using the GPS track as an underlay, sharp turns are not recorded accurately. I have mine to drop a trackpoint every 1/100 of a mile (the finest distance-related tracking it will do). It may do better if I tell it to drop a point every second, but that would be a much larger file. May not even be able to hold all the trackpoints in one file for a half-marathon course.