When is the best recommended time to spay or neuter your dog?

It used to be done early because people would get their dog license cheaper if the dog was spayed or neutered at 6 months when they came into the vet. And the spca tended to spay and neuter at an early age to decrease pet population problems. 

However what has been discovered, with scientific evidence behind it, is that waiting until the dog is full grown is best for your dog. Meaning that the growth plates of cartilage at the end of their bones, that get them to reach their size, have closed. The closure of those growth plates is under the influence of their hormones: estrogen and testosterone and if you spay or neuter before that happens it can lead to joint problems. 

It is recommended to spay and neuter your dog when they are full grown. And if they are a girl pup to wait till after their first heat completes.