Actually I had forgotten about the Grob-115C2, which uses the O-360 engine, again a bit of leaning on taxi would remedy any fouling if the engine was healthy. For the engines on the way out a bit more of a run pre take-off might be needed to clear plugs. Never heard of the idea of running at 1800rpm prior to shut down, really not sure it would prevent that much fouling as it would just start building again the next taxi out, as the engine would be at it's coldest then. I do remember sometimes quite a long idle warming the engine as well after start if it was first flight, as LB said some leaning would help it warm as well as preventing fouling.