Soaking regs in water that other gear has been rinsed or used for soaking is just soaking them in dirty water. If you are talking gear used in saltwater, soaking them is just soaking them in that.Interesting - what is the technical reason behind not soaking them? And is simply rinsing the regs the best way of preserving them?
I'll look into getting a different (cheap) BCD for the pool since we don't have many spare rentals once the students have been assigned their gear for the course.
Rinsing gear with clear freshwater and working the knobs and buttons helps to remove the contaminants.
Soaking regs unpressurized risks flooding the 1st stage if a purge button is pressed or the DUST CAP (not waterproof cap) is loose or dislodged.
Running fresh water is the proper way to rinse regs. Soaking them is a good way to risk unnecessary damage and excess wear.