Yeah, my training has been to only consider feathering the props for a gear up if you know the airplane well enough that you think you can get them horizontal - but a low priority in any case. If the engines are shut down, and three blade propellers are stopped, or gently windmilling, the hurt to the engine will be the least it can be for the circumstances. Aim for a hard surface runway, not the turf, for the reason stated above, you'd rather slide and coast to a stop, than to dig in some part of the airplane and spin at around or flip it.
When I one had a gear failure, in a 310, and thought that a gear up landing (at Toronto YYZ) was a possibility, we discussed foaming the runway. The best wisdom was to foam the runway well down, so I could touch down on the dry, and slide into the foam, rather than land in foam, or accidentally overfly it in a poorly judged flare. Happily, the gear mysteriously free fell while I fought with the (non functioning) emergency crank, so a normal landing was possible. Though I did land three green, my mistake was taxiing in to the apron. I later was reminded that I should have shut down on the runway, and had the gear physically locked down, and had it towed in.
I've had to land amphibious floatplanes one side up, the other side down a couple of times. Happily, landing gently on the ice that way is a non event.