Hello,
The common thing taught in schools ( when woodwork was actually taught!) was.to lay the plane down on it's side. This is pretty much ingrained in everybody's consciousness, that it is rarely questioned. It might seem like a small thing ( in the scheme of things, it is) but I think this notion is wrong.
Firstly, your bench should be kept clean of anything that will ruin your work. You do not want to spend time making something, only to have to repair it because of a big scar caused by a foreign body. If you use your bench for non-woodworking things, use a sacrificial top. Once we have the (idealistic?) situation of the clear bench, putting a plane down on its sole is the safest for it. You will not dull the blade resting it on a wooden bench top, nor harm the sole but on its side, it is very easy to run into the exposed blade or bash the sole with another tool, another plane in use is the likely candidate. It might not happen often, but when it does, it is better to bump the side of the plane than the bit that matters.. Putting the plane sole down on a stick or two is fine, I suppose, but I don' see a benefit and it only causes bench clutter. Storing them that way in the tool cupboard is good, though, as it keeps the sole off potentially damp dust etc. and keeps some airflow, which helps stop corrosion.
Mike.