I always preferred performing the topping checks on the ground whenever possible, however, not all aircraft would permit that method. In flight topping could become interesting especially when at the moment you record the rotor RPM droop the engine not under test experiences a governor failure and everything gets a bit exciting.
Yes, the 212 had a part power stop (I think it was called) that allowed that, but I think it was for setting up and not for final verification.
There was an incident at Aberdeen Airport, but I can't find the report because the internet is saturated by more recent accidents of the 332/225 now. Engine replaced with bolts not tightened at the high speed shaft MGB connection (multiple shifts). During ground runs, the connection welded itself from overheating from friction, so was not evident. Commander elected to do the topping check in the visual circuit, with a 700 ft cloud base. Commander elected to manipulate the speed select levers while also the handling pilot. High speed shaft failed with the other engine at idle, on the downwind leg. Commander managed to get it onto a cross runway.
Sorry for thread drift.....