Are you in contact with each and every single owner of your past puppies? Do all of them still have their puppy? Are their puppies well cared for and trained? If you answer "no" to any of those then you cannot guarantee good homes to any puppy, past or future, and should not breed for that reason alone. 3 out of 4 dogs wind up homeless, abused, neglected, in shelters, or dead long before they hit old age. Breeding without stringent home checks and strict, enforced contracts that require them to return the dog to you at any point in its life if they can't keep it (enforcing large financial penalties for failure to spay/neuter and for rehoming without your consent) guarantees that at least some of those puppies will be dumped at shelters eventually, or abused, or neglected. This is a fact. Your friends and family can pinky swear all they want, but most of those shelter dogs came from the same kind of people who wanted a puppy so very bad, and wanted that dog forever and ever.
Mutts are euthanized every day in shelters. Is it your problem? No, but you'll only contribute to it by breeding. If even one of your pups winds up in a shelter or abandoned then you're only part of the problem.
What are your dogs' OFA, CERF, and bloodwork results for liver and thyroid? Has a specialist given you a perfect bill of health for their hearts? If not, you have no business breeding. What about genetic health testing? Everything I've listed checks for things that are not immediately apparent and often are missed on a routine veterinary exam. Your vet saying "yep, they're healthy" is not good enough for breeding. Pumping out even more questionably bred puppies of questionable health from parents that were never properly health checked contributes to the problem, believe me.
Breeding mutts is irresponsible. Breeding badly bred purebreds is irresponsible. Breeding for ANY reason other than to produce performance working dogs with proven working drives and show/conformation standards to ensure breeds don't disappear completely and can still do what they were bred to do is irresponsible. While I agree that the shelter overpopulation isn't my problem, I'm not about to add to it by breeding my mutt, adorable though he may be.
There's more to the problem than "I have homes so what's the big deal?" The fact is you underestimate how obscenely ill-prepared and irresponsible even our own family and friends can be. You underestimate just how many people get a puppy because it sounds like a good idea but honestly think they come knowing how to poop outside and how to sit on command. You'd be surprised how many people give a 6 month old dog the boot because they can't or won't train it because they didn't realize it was that much work.
Furthermore, you clearly don't understand basic genetics. I get the impression you're one of those "my dog is sweet and I want to pass on her good genes" breeders. You breed with no regard or knowledge to recessive health defects or how the pups will also inherit traits from the stud dog. This is why it's exceedingly important to get proper health clearances and have both dogs pass a proper temperament test administered by a professional. Failure to do so means you only contribute to the problem, whether you want to admit it or not. By ensuring the health of your dogs through proper temperament and health testing and doing very strict, rigorous home checks, you greatly minimize and even eliminate the chance of those pups winding up anywhere BUT a permanent, happy home. I don't care if your dog is a mutt or not, but I do care about your assumption that because somebody promises they can care for a dog that that means they actually will.
[Add] You'll further directly (yes, directly) contribute to the shelter problem when somebody stupidly lets THEIR mutt that they got from you breed and produce more puppies. You have no idea if those pups will wind up in good homes or not, nor do you have any say in the matter, but it will still ultimately be because of you.
[Add] Oh yeah, and also check out the pet section of your local Craigslist. Look how many "free to good home" or "needs new home for small fee" dogs -especially puppies-are there. I live in a rural area and at least once a day (usually several ads a day) there's a "I got a puppy last night/two weeks ago/a month ago and the landlord found out/dog requires training/i didn't know my dog-aggressive breed would be aggressive with the puppy" crap. Guess what, each of those puppies "went to good homes" only to wind up on Craigslist a few days or weeks or months later.