you didn't vote... do you use hydrogen peroxide instead of bleach? will boiling the rocks to get them clean damage the air pockets of the clay preventing it from retaining moisture?
I've tried H2O2, but I think I am going to give boiling a shot. Boiling is cheap, and unlike bleach, I don't have to worry that I haven't rinsed it all off.
(I suspect the amount you have to clean would be a factor)