There are some here who show and breed who do recommend waiting until after first heat. It is for skeletal reasons. If we got another dog, I would consider it.
We spayed all of ours before first heat. Two at 6 months and one at 9 months. Each was 3 pounds on spay day.
My smallest who was spayed at 9 months was a breeze. She did the best of the 3. I only had one who had to be taken back the next day as she was not managing pain well. Other than that, all did well.