RE: nitro fuels
All your glow fuels are going to create a certain amount of carbon build up.
Personally, I wouldn't worry about it. As long as you take care of the engine, it will most likely need a rebuild before/if the carbon ever becomes a problem.
I would be more concerned that the fuel have some castor oil in it. The castor, even though it will leave a hard to remove varnish on the outside of the engine/muffler, will protect the engine in the event of a lean run.