I do not agree with Onan. I never have, but I have only been taking care of other people's (usually very expensive) engines since 1960.
If you take care to drain (or at least run the) carburetor dry, that is most of the issue. I have never seen significant internal corrosion on engines that were merely "bottled" up when shut down. This was all on tidal water (salt). If you are worried, learn to "fog" the engine. (This means introduce a cloud of oil as you shut it down.) If you think you need to warm the windings, the varnish that needed that was replaced before the second world war.
Keep in mind that the people that want you to run the engine regularly are also the ones that will want to sell you parts that got thermal cycled to death.
Matt_C