There are pros and cons to both, though the earlier spaying is definitely more common. Allowing the dog to go through even one heat can increase the chances of cancer down the road. But allowing the dog to go through at least one heat allows the dog's hormones and body to properly mature the way that nature intended. Some people say that (especially with larger dogs that still need to grow a lot at 6 mos old) that cutting off the hormone cycle might effect the joint growth possibly leading to joint issues down the road. This is especially important with service dogs, sport dogs, etc, but also something to think about with dogs prone to dysplasia. It seems like a very valid theory, but I don't know if any studies have been done. However, it can definitely change a dog's overall structure- male dogs tend to be taller and thinner, less muscular, if they have been fixed early. My thoughts on this have always been: who cares?
Especially since my dogs are rescues or mutts and I have no idea what they should look like anyway!
I have always had my dogs fixed (spayed usually, most of my dogs have been female) BEFORE their first heat. My Bushi was spayed at the shelter I adopted her from, at 8 weeks, which is common for shelters!