When I'm viewing I have barlow then diagonal then eyepeice - main reason is to reduce the amount of weight hanging out the end. By having the barlow first the majority of the weight of the barlow is inside the OTA/microfocuser thus reducing overall weight in mid-air = less stress.
When imaging I dont use the diagonal at all - the less glass for the light to pass through the less chance of something dodgy happening to it - dirty lenses etc
