Instead of the event approach, our suggestion is to use Conditional Formatting
with Style Edges Padding Bottom set to 0.2 in and the following filter:
| = RowNumber("MasterReport2") Mod 5
| = RowNumber("MasterReport2")
About your approach - using the report item's definition counterpart is considered a hack with unexpected results. The sender of the event is always the processing counterpart of the data item. However for the current situation it's not applicable.
Check out the following help articles:
the Telerik team