My current older dog (a rescue chihuahua of all things) is a registered therapy dog with Happy Tails, and we do a lot of visits at hospitals and nursing homes. I say this to state my creds
When I went looking for an SP (I grew up with the breed-the chi was a "I must have him" anomaly lol), I looked for breeders that did all the testing, but you can't really expect dogs to all come out with the therapy dog temperament, regardless of breeding.
If you watch the movie about service dogs, "Pick Of The Litter", you will see how difficult it is for dogs to make it as service dogs-and they specifically breed for service dog temperament!
Therapy dogs obviously have a much lower (yet still stringent) threshold for passing, but my point is that you have to look (and sometimes wait) for just the right puppy.
And even if your puppy tests "right" on the Volhard tests, he/she still may not grow up to be a therapy dog. Mine at 6 months is perfect in every respect for therapy dog work, except he is going through a phase (or not) where he is reserved around strangers.
This is not so good for therapy dog work, lol, but is *excellent* for me being in situations where he and I are alone if my husband is traveling for work. People who used to come up to me and harass me on the street give me a wide berth now (we live in the city). We just breeze on by
My SP is super well socialized, btw, he's on elevators every day with strangers, he walks city streets, he plays with dog friends in the burbs, he knows which valet guys have dog treats, he sails on boats, he runs along the beach, and he just completed beginner obedience with flying colors. He just isn't a fan of people in the elevator looming over him and wanting to pet. He's polite about it (because I give him treats like crazy and he's got a solid temperament), but I can tell he'd much prefer to admire people from afar, lol.
So, he may evolve into being cool with dog loving strangers coming up to him (because he's gorgeous and people are so attracted to him), or he may be more of an introvert who would find therapy dog work too stressful (it is stressful for dogs, and my chihuahua and all the dogs he works with are always tired out after our visits. Humans are too-it takes a lot out of you and your team emotionally). I'm totally cool with however he wants to roll.
My point is only that there are no guarantees.
Not in a million years could you have told me that a chihuahua would be a shining star of therapy dog work, ha ha, yet I've got one.