I don't believe there are any hard and fast rules. In the wild, they will mate with whatever animal they encounter that is willing. They don't do background checks and genealogical charts.I believe species that would suffer from line breeding or in breeding have been designed with social structures that prevent such matings, or at least prevent them from occurring too frequently.
My only personal rule about such things is to avoid it with morph lines that are genetically "weak" to begin with. Specifically, spiders and caramels. In those cases, close familial breeding may exacerbate the wobble or kinks.
There haven't been any proven "lethal" genes (That I'm aware of)...although on the fringes of the morph world, there are some mysteries that you're unlikely to need to worry about as you start....such as the super form of the "hidden gene woma" morph, called The Pearl. Some folks wonder if a "super spider" is lethal, but there is no proof of this.