well you don't want to get water down there so there is that. A dirty drum, all that really might do is slow down the loft a bit, ie give a bit more friction so the beans don't loft higher, but unless you have huge gobs of crud built up, I don't think it'd hurt too much really. Since the heat is not radiant from a lamp like a Behmor, I don't see where it would affect the roast too much. If you wanted you could try it but I dont think it'd cause too many i ssues.
Aaron
