Hello, I'm Dr. Deb and will do my best to help you today.
This is an interesting question that you've asked since there is some debate in the veterinary versus breeding versus rescue community about this topic and the answer might depend on who you ask.
I routinely recommend spay and neuter at 6 months of age for both dogs and cats. If Minnie is very small (weighs <2 pounds for example), then I might wait until she has gained another pound or so although I do know that some vets would spay her at this age regardless of her weight.
For the larger and purebred dogs, I'll often suggest a first heat since there's some evidence that to do so will reduce the risk of ligament damage (such as ACL injuries) and possibly some cancers such as hemangiosarcoma; however, these issues aren't a commonly seen problem in this breed.
Breeders will often suggest waiting until after a first heat for those females who are prone to urinary incontinence after they've been spayed; again, though, this problem isn't commonly seen in this breed either.
Shelters and rescue organizations will spay as early as 6 weeks of age (to reduce the risk of over-population) but personally I think this is too young.
Every vet has their own protocol as to the "optimum" age to spay a female but 6 months is by no means too young and the age that most vets will recommend this procedure. Since smaller females tend to come into heat sooner than the larger dogs (often at 7-8 months of age), spaying before a first heat is often preferred primarily because the risk of breast cancer is reduced if we do.
I hope this helps. Deb