Hi BaldingEagle;
In the US the reports of level and reaching or leaving an altitude when in radar contact are not needed, however most of the airline part one manuals require the pilots to tell us. Now, if you are checking on from one facility ( not from one sector to another) then we are required to confirm your mode C for our facility. Other than that, we just use your mode C unless we REALLY need to use altitude separation and we don't want to wait the extra couple of seconds for the radar to let us know. You know how it is when you are running things tight and seconds count... <G>
regards