In the US it is not necessary to report leaving or reaching an altitude unless requested by ATC. Indeed, such reports are discouraged by controllers. I believe that the only exception is when an aircraft is issued a discretion descent.
Sorry RadarKing but that is wrong.
AIM 5-3-3
5-3-3. Additional Reports
a. The following reports should be made to ATC or FSS facilities without a specific ATC request:
1. At all times.
(a) When vacating any previously assigned altitude or flight level for a newly assigned altitude or flight level.
Don Brown