There are no easy or standard, "cookie cutter" answers.

I work with 2 breed-specific rescues - we'll call them Rescue W and Rescue S. Both have a policy of not adopting any dog directly after it is surrendered or pulled from a shelter. Instead, the dog is fostered and evaluated for at least 5 weeks. This policy alone has greatly reduced the number of returns. For Rescue W, 1 dog of 34 adoptions in 2009 was returned. For Rescue S, 5 dogs from 64 adoptions were returned ....only one for aggression (establishing an actual bite history) and it turns out that particular dog has epilepsy and he's doing very well now with medication. A vet check, especially if the dog has been in the new home for some time, is always important!

Rescue W is more "lenient" with regards to aggression, especially aggression towards strangers, because the breed is known to be at best aloof with strangers. Because of the breed's tendency towards guarding behavior with a very strong work ethic, we place dogs very, very cautiously and evaluate potential homes very, very carefully before adoption. 95% of potential adopters are denied because they have no prior experience with the breed ... this is probably one of those rescues that people gripe about on Internet forums ;P

If a dog is returned for "aggression", we want to know as much as possible about the incident. Aggression that occurs in the first month or aggression towards non-family members, especially when it is only an aggressive display, is usually discounted, meaning that the rescue will offer the services of a professional (free if it's me, discounted if it's one of two other trainers) or the dog will come to me or another experienced foster family for further evaluation. Territorial aggression can be managed successfully and this type of dog can often go to another home with a more experienced owner.

Rescue S works with a sporting-class breed, so aggression is seen as a much more serious fault. ANY dog who is returned is placed with an experienced foster family and efforts are made to duplicate the situation where the problem behavior occurred ( except the dog being "hyper" ...it chaps my cheeks to deal with people who return a dog because he's too "hyper" when anyone knows the breed is high-energy). Rehab is generally successful with regards to things like food aggression/resource guarding, especially when a new owner is committed to following a protocol. Because this breed is generally known for its sunny disposition, territorial aggression is quite uncommon and a dog displaying territorial aggression even without a bite history is generally euthanized during the first fostering period.

It is very uncommon that fear aggression not manifest during the initial fostering period. In both rescues, dogs displaying fear aggression are generally euthanized after being evaluated. The amount of time and skill needed to successfully rehab this type of dog is simply too great. Yes, it is very sad, but the liability should an aggressive dog be adopted out again and injure someone is too great and the pool of suitable homes with accomplished owners is too small to do anything else. No one walks into a shelter or rescue and asks for the biter.

Shelters and rescues do a disservice to the public when they hide problem behavior instead of doing the responsible thing. It's all well and good to say that all the dog needs is a quiet environment with no kids, but single dog-lovers move, get married, have kids, have nieces/nephews/neighbors etc., etc., etc. The hard truth is that I can foster 5-10 "normal" dogs who go on to make someone an absolutely wonderful pet in the time it takes to rehab one problem dog.