Rule #1: breeding siblings is worse than child to parent
Rule #2: Cross out proven genes as much as possible
That's about it. I haven't personally seen any issues caused by inbreeding. All the issues that were supposedly due to inbreeding (caramels kinking, spiders wobbling) have been pretty much accepted by the community as being caused by the morph gene, thus impossible to breed out.
With morphs, we're dealing with a fairly select group of animals. Even with WCs coming into the country, the focus is more and more on morph combos. This is a fairly strong limit on the genetic diversity, so inbreeding is inevitable even without direct sibling to sibling or parent to child breeding.
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