A dog's temperament is the combination of mental, physical, and emotional traits or natural disposition. This is who your dog is; and your dog's core temperament comes down the genetic lines from her ancestors. When you go and see a puppy; you want to make good and sure that Mom and Dad have nice temperaments. That is not to say that somewhere down the line; perhaps a great Grandpa had a strange temperament, that can rear it's head at some point. But at least if you know Mom and Dad have a good temperament there is a good chance that your puppy will as well.
Temperament is #1 on the important list for me; but not for everyone. Many breeders love a certain look or physical attribute and will overlook a not so fabulous temperament to get it. A structurally sound dog is very important but a dog will short legs, a low tail set, crooked teeth or any other small fault can be overlooked by the average guardian. Temperament on the other hand can make or break a dog; it can be the difference between a great dog and a very difficult dog. I don't know how many "poor temperament" dogs I've met that had parents with the identical temperaments. Asking new guardians about the puppy choosing process; the parents etc and they say they couldn't meet Mom or Dad, hmmmmmmmmmm.
Too many dogs are bred who do not possess stellar temperaments. Many dogs that are spill overs from performance lines end up in the average Joe household; and most average Joe's can't deal with a drive level that is off the charts. Many people purchase a puppy from hunting lines; staying away from show lines. Their thinking is that they don't want a poofy show dog; they want a real dog. Well; real is what they get, real drive and an energy level to match that of the energizer bunny combined with an Olympic athlete. These dogs often become bored and highly destructive; had the guardian only known what they were in for.
Some of the greatest temperaments I've seen were in mixed breed dogs. A good temperament is a good temperament and something I love to see in any dog. One of the best I've ever seen was in a huge st. Bernard mix. He had lived on the street for the first part of his life; he was amazing. There is truly nothing like a sound temperament; a dog with a rock solid temperament should be put on a pedestal and bred.
Temperament is core; but a temperament can be altered somewhat. A dog can be taught to be better or worse by environment. I do not believe that a dog who is predisposed to a bad temperament is doomed. Dog behavior can be modified; you may not be able to change the core temperament of a dog but you can alter their reaction to the world around them. Fearful dogs can learn to fear less; dominant aggressive dogs can be changed by an experienced guardian.
Some dogs may seem as though they don't have great temperaments due to the lack of socialization. Given this hand in life a dog may never be all the dog that they could have been if placed into a different environment. So good dogs can go bad and bad dogs can go good if placed in the proper living situation. This is why I believe strongly in temperament testing; it is not the final word at all but it helps to place a puppy in the environment that they will most thrive.
Unless you and others can say AMAZING about your dog's temperament; then please alter them.