This is a very controversial topic amongst breeders. It's also something that a lot of people don't fully understand and unfortunately, will base their opinion of breeders on this decision alone.
Here, I do breed my girls 4 times consecutively. Many top dog reproduction specialists have recommended this and said it doesn't do a female any good to skip a cycle after each litter. It can actually be worse, especially if you do this after every litter of puppies. (The first cycle should be skipped to allow time for the dog to mature and get health testing completed.) The health and well-being of my dogs is my top priority. If my vet or I ever feel that one of my girls needs to skip a cycle, I will absolutely do that. Since they live in my home I'm very in-tune with them and can sense things, just like parents do with their kids.
A dog's uterus ages with each heat cycle regardless if she's bred or not. When a female ovulates, her progesterone lasts the same length of time whether she's pregnant or not.
Pyometra is dreaded amongst breeders and pet owners. Pyometra is when a uterus fills with pus and can be deadly. An intact female is at risk for getting this. It's caused by hormonal changes that occur when female dogs get their heat cycle. Pyometra typically occurs 2-8 weeks after a female's cycle. The way vets treat this is by spaying the dog. "An idle uterus is the ideal setting."
Let's say a female dog has her first litter at 1.5 years old. If you skip a cycle after every litter until you get to 4 litters, she'll be at least 5 years old when she's spayed. Breeding back-to-back puts her at about 3 years old when she's spayed. That's why it's better to breed her back-to-back and spay her young.
https://www.akc.org/expert-advice/health/preventing-and-addressing-pyometra-in-breeding-bitches/
https://www.revivalanimal.com/pet-health/heat-cycle-basics/learning-center
https://www.gooddog.com/good-breeder-center/webinar-with-dr-hutchison
