Actually, it is safe to breed back for 5 generations, but not earlier.
Personally I can't agree with the concept of using inbreeding, or line
breeding for that matter, to the extent as has been suggested.  I know
it's an accepted practice in show dogs and cats, and I don't like it and
believe it's a problem for many breeds.  I feel line breeding has its
place at times, but while "in -breeding" generation after generation may
bring about a "standard" of a type you want, the devastation regarding
health issues far out ways any good.  Having said that, this is why some
breeders do it.
 
Out crossing can be an excellent way of avoiding problems showing up
in the immediate offspring, which is why I assume many or most ferret
breeders have utilized it extensively.  Unfortunately, it also passes
along any deleterious genes present without expressing them and alerting
the breeder to their existence.  By the time two carriers are mated and
the breeder is aware that the problem exists, it may be difficult to find
one who is not a carrier.  The original breeder has avoided the problem,
but has also ensured that future breeders of his/her stock will have to
deal with it eventually.
 
The key to diversity is to have a number of breeders who have
concentrated on line breeding their stock to bring problems out in the
open and then work to eliminate them.  Thus, if a problem surfaces in
one line, there are other lines which can be out crossed to which do not
carry it.  Continued out crossing eliminates diversity; it does not
preserve it unless new and unrelated stock is continually brought in...
which may also bring in serious problems of their own, possibly ones
which had not previously been seen.
 
Closely line bred stock, which has been bred with the emphasis on health,
may carry one or two problems but carriers can be easily detected (they
produce affected offspring) and eliminated.  A ferret which is the result
of several generations of out crossing may very well carry most or all of
the problems in his/her background, particularly those having or carrying
dilute colors, and thus be a genetic time bomb just waiting to explode.
 
The key to it all is the priorities of each breeder.
[Posted in FML issue 4957]