I read up on inbreeding in one of my snake books. I can find the name and page number later if you so wish.
There was a case of someone from Germany I think, who bred his snakes eight generations brother-sister. And had no ill effects. In fact he said that the fertility increased and he produced healthier, larger animals. This is just what I remember, so it was worded differently. But I think it proves a point.
Inbreeding makes the gene pool smaller, but it is only if there are hidden defects that there are issues. It increases both the good points and the bad points. Ie hereditary defects, or a desired colour mutation.
Personally I have not much of an issue with inbreeding, but I would never go further than to breed back into direct 'family' twice. Not with me personally, but if you happen across a brand new recessive gene, how do you prove it out when the only know animal that carries the gene is the 'child' snake's sire and dam?
I'll find that page and post it on here when I get home.