What breed are your dogs? Suppose it really makes little difference. Speaking entirely from my own personal experience, my females that were spayed before their first heat cycle have never had a problem with incontinence and they are all between the ages of 1-year up thru 10-years old. However, I do have a female that was spayed after having litters of puppies, that wets herself at the slightest excitement and another female that was spayed after having had one-heat season, that unless I make certain, she has emptied her bladder right before bedtime, will undoudtedly leave a pool where she has slept. So now, given the option, I would spay prior to the first heat season, taking advantage of the benefit of preventing mammary cancers. So I wonder, if perhaps the incontinence theory is
as your vet said, 'unproven'; or perhaps my experiences are just an exception. So I think you are going to find opinions vary quite a bit on this subject.

I would go with my vets recommendation. (By the way all my girls are Westies)