USPS PO Box Address Format for Return Address

[Miles Stanley]

i just opened a PO Box and i asked the clerk(s) there if when i put the PO Box address if i could write it as:

xxx Post Office Address #XXXX

City, State, Zip

or if i HAD to write out

P.O. Box XXXX

City, State, Zip.

...and they didn't know for sure.  

I want to use the '#XXXX' format bc to me it looks a little less PO Box-y, but i don't want to risk my letter getting kicked back needlessly.  Does it matter for direct mail??

Thanks,