How far, family-wise, apart should balls be before breeding?

Aka, when breeding balls how far apart should they be (aka cousins, second cousins, etc.)? The easiest way I could say this would be: Would it be a bad idea to breed snakes with the same grandparents together? Great grandparents? How far must you take the line before you could breed back into it?

Check this link out: http://www.ball-pythons.net/forums/s...ght=inbreeding

The first page discusses line breeding.