Incubators are such fantastic breeding grounds for bacteria, it makes sense to be as clean as possible I suppose. HesterF - you're right that the red ring is an indicator of early death of a fertile egg, but my understanding is that this is often caused by bacterial contamination. I'm sure someone more knowledgeable will be along to confirm or dismiss shortly.
I bought a load of eggs from fleabay this week, but one box arrived with the eggs covered in sh*t, egg yolk and bedding

. I disinfected all of the eggs before setting, but I've deliberately set them in a different incubator just in case.
Too fastidious? maybe. However, if it only takes one bad egg to spoil the incubator......