It depends.
On the newer radar systems with ARTAS, the computers can calculate ROC/ROD as part of ths short term conflict alert(STCA), and may provide spurious "level bust" warnings to the controller.
This happens because the radar systems are configured for IFR commercial traffic and as such operate within pre-defined limits. Traffic climbing through 7000' at 4000'fpm may trigger an alert with cruising traffic at 10,000' because the computer predicts that the climbing traffic may bust the level cleared to..
If you're doing aerobatics in controlled airspace or over an aerodrome, I'd prefer you have mode C off. This is because another aircraft will not come near you as the controller shouldn't allow it.
If you're performing in un-controlled airspace, leave mode C on. It will aid the controller in providing traffic information to the pilot who after being warned of the activity still wants to fly through (there's always one). Also useful as an aid to collision avoidance for TCAS equipped VFR aircraft in uncontrolled airspace.