I install my engines at 135* so the muffler exits centered with the bottom of the plane. Very easy to hide, and deflects all the mess away from the plane.

The idea of an engine running poorly inverted I believe has to do with the glow plug being perfectly upside down such that fuel entering the engine will flood it. With a good plug, proper tank position, and a properly tuned engine, I don't think this would be an issue. Going out on a limb here, I bet that those who do have issues with inverted engines have one or more of the above setup incorrectly.