How accurately do I need to maintain my altitude while VFR-on-top?
Considering, as a pilot, you should be able to maintain an altitude with Less than a 100 foot accuracy, I would say that you should be doing the same when VFR-on-Top. If you change (vacate) that altitude for any reason, you should let ATC know. They are still maintaining aircraft separation. They need to anticipate your current and future position. If it’s a brief departure from the previously reported altitude (say for instance, due to turbulence), reporting the discrepancy is not important if you return to the original altitude immediately. Though, you might want to give ATC and/or FSS a PIREP.