I generally run my O2 down until it can't sustain a neutral flame for whatever I'm doing. Then I change out the bottle. That will usually leave me with ~30 PSI in the bottle.
Generally speaking you should never bleed a bottle of any type completly empty. The reason for this is that a bottle at atmospheric pressure will form condensation on the inside of the bottle which then pools as water in the bottom... and forms rust... which is not good in a cylinder. At long as you maintain the pressure somewhat above atmospheric pressure it's not going to condensate, at least not as much.
If the supplier then wants to empty or purge the bottle, that's up to them... but ast least it's being purged and refilled right away so it remians under pressure most of the time.