It depends very much on the type of aggression and the owners' situation. If the dog is DA, but is an only dog, with a fully fenced yard, that is easily managed. If the dog is aggressive towards kids, but the owner has no kids and can keep the dog safely contained - no problem. If the dog can be safely managed and owner is willing to manage the dog, the dog does not need to be PTS.
If the owner is unable or unwilling to manage the aggressive dog, does not want to work on improving the aggression, or doesn't want to keep the dog, it is probably best to put the dog to sleep.
Here is an example of a dog I think should be put to sleep. Someone in my neighborhood recently adopted a pitbull from a pitbull rescue. The adopter was not warned that the dog was DA. On the first walk, the PB attacked a senior dog, going for the throat and causing $2000.00 worth of vet bills. The PB redirects to the handler and bites, when it cannot get to a dog. The rescue does not return calls. The adopter does not wish to keep or work with this dog. IMO, this dog should be put to sleep. There are many kids and dogs in my neighborhood. The risk of serious injury is too great.
Natty Boh 6-27-12
Annie 1998 - 8-2-12 RIP