Until research proves it, there is no substantial evidence that 428 or 432 are better than 440 for therapy or other purposes. In fact, for those patients with perfect pitch, tuning low could be annoying and stressful, which is exactly what we are trying to alleviate with patients. Those who claim "the ancients tuned to 432" - well, which ancients, and how ancient, and what instruments, and what is the evidence?
Here are the facts:
1. mathematically, it makes no difference. What IS important is relative pitch, mode and scale. See my book "Singing the Universe Awake".
2. Historically, pitches have been constantly going up to accommodate larger and larger spaces/audiences (higher pitches give the illusion of more volume). In Renaissance times they usually used 417. So any culture older than that would not have used a pitch higher than that, but a lower one.
3. Most tribal cultures had no tuning standard. Only developed societies' musical scholars did.
4. Standard pitch is decided upon by the majority because most professional musicians need to be able to play together. It is not ordained by any universal force. There are specific resonances and tones that are interesting, such as 7.23 Hz, the supposed frequency of planet Earth, and 7.8 Hz, the average brain wave frequency of people in deep meditative states and of Earth's electromagnetic field.
The claim that tuning lower is somehow more therapeutic was invented to give us a reason to feel special and different. That does not behoove therapeutic musicians, since our focus is not on ourselves but on our patients. That said, if you genuinely like the sound of a harp tuned lower, there's no reason you should not do it, unless you play in ensemble. To your patients it will not matter.