I would only clean mine out on the inside if I had a reasonto other than getting rid of ashes and cleaning water pans and such. My sister had a little sign in her kitchen that said "My kitchen is clean enough to be healthy and dirty enough to be comfortable". She was a RN so I took her word for it.
I have a large drain hole in the backside of Puff, so if I decide he needs a bath, I fire up the pressure washer and go to town. I open the drain hole and rinse away the grease and residue. Puff doesn't get many baths though unless I am feeding large quantity of strangers. If I am cooking for myself and my friends, I just let him cook off any "contaminants". The smoke stack usually runs over 300 degrees when I first fire him up with a full load of lump. The temps stay there for 30ish minutes while I regulate and get everything down to a comfortable 225ish. I figure any lingering germs are pretty well zapped with 300 degrees.
Unless you are worried about appearances, I wouldn't sweat it to much. Personally, I prefer having that lingering smell for as long as possible!