Once you report reaching VFR-On-Top and the controller reclears to to
"Maintain VFR-On-Top" he isn't responsible for separation. After that he
doesn't really care much what your actual altitude is now or might be later
(beyond verifying your Mode C for traffic or idle curiosity).

Maybe so, but you're required to report changing altitudes when flying
VFR-On-Top (at least, per the AIM)...

--
Cheers,
John Clonts
Temple, Texas
N7NZ