All those things are myths, I on the other hand have seen dogs (used to work in a vet clinic & for a shelter) did if diseases that could have prevented by sputering.
When bear was done showing I neutered him & he is 13 & going strong
, I think he will outlive all of us.
The only thing I would let a female go they a heat for is if she has a recessed vulva or something like that.
I will never be convinced that, other then Showing or medical reasons, sorry but the dog should be fixed.
Sputering has proven to prevent reproductive cancers when preformed early enough (IMHO before the 2nd year).
Your childhood dogs might have been lucky.