Good point, I've never really thought about it, but you've got me thinking. I imagine that it depends on why the engine fails - catastrophic major component failure, like a crank / complete seizure then I guess not.
The props will normally spin fairly fast with no power, in fact if you 'fail' and engine in a twin, by switching off the fuel, then its difficult to actually see which engine has failed by looking at the prop. Of course when you feather it, it becomes obvious, but to restart you normally put the prop forward, give it a bit of a kick with the starter then the windmilling does the rest, so I assume it windmills pretty near or above idling speed.
EA