Breeding defective animals

[Wilko]

After touching on this on another thread, I thought I should start a proper thread in the appropriate area

Where do you draw the line between breeding "defective" animals? A slight tail kink, a severe spinal kink or a major head wobble / balance issues? Would you breed for the colour of the animal regardless of any visual defects (because it's that good/expensive a morph) and then hope the offspring are fine?
I'm not targeting any specific species/morphs, I'm just hoping to hear some people's opinions on this