Congratulations on the new members of your family.
You have asked a difficult question that could vary somewhat for different breeds. II will be interested in the answers you get. Hopefully, Doug will offer his opinion.
I believe one school of thought suggests spaying after 6 months, but before the first heat cycle. (Some breeds my have a heat cycle before 6 months.) There is evidence that this reduces the risk of some cancers.
We have active dogs that compete in agility, obedience, rally, and herding - for the border collies. They also track.
Because of that, we are concerned about orthopedic consequences of early spaying and don't spay our ******* until their their growth plates have closed.
I will add that, with one exception, all of our golden retrievers have died of cancer, but they have all lived to at leaf 14.5 years old, which is a couple of years longer than the average big dog.
Tom
2015 Meridian 36M
2006 CR-V toad
3 golden retrievers (Breeze, Jinks, Razz)
1 border collie (Boogie)