Waiting until after the first heat to spay has NO medical documentation behind it. It's basically an "old wive's tale" of the doggie world.

There is an argument that spaying later, after growth is complete, is better for dogs, especially large breeds. While this has never been proven, it at least has some basis in logic. Waiting until after the first heat has NO basis in logic, as the dog is still growing at six months.

I, personally, would be very, very disappointed if my vet believed this.

Ovaries and uterus are both removed in a spay. Ovaries are often left in human hysterectomies to avoid calcium loss that leads to osteporosis, as well as the onset of symptoms produced by reduced estrogen levels, such as chin hairs for example, that most human females are NOT happy to see. This isn't a problem with female dogs, however, their chins are already hairy. Hence .. out with the ovaries!

Some dogs do suffer spay incontinence, but it is a fairly rare condidtion. I have never had a dog with spay incontinence, and I've had dozens and dozens of female foster/rescue dogs spayed. If it does occur, an estrogen pill once or twice a week solves the problem in the vast majority of cases.