I agree with Matt.. 6 gens is usually when a breeder will bring in new blood, but some will go up to 9 depending upon their goal.
If you want a particular color or pattern, and have a good pair from a spawn then you would want to breed those two siblings together to carry on those genes/colors/pattern. If you are trying to achieve something that maybe only one parent has like color, pattern, etc.. then breeding a baby that has the most of what you want back to a parent with the qualities you want, that is the way to go.
Inbreeding won't cause the problems you see when you are responsible when doing so - just like with any animal, can inbreed to a point. With fish in general the rules of nature are a bit lax because they usually don't have that big of an opportunity to find new genes as they are pretty much stuck in whatever lake or river they reside in.
My main goal is health when breeding.. why I don't power grow the fry - pretty much let them grow a natural way. Sometimes I'll get some that grow faster than others, etc. But I don't go to the extreme of feeding 6 times a day, daily water changes, etc. I want them to have a longer life span if possible, as well as being healthy.