I thought i recalled that the post office required either PMB or # as a prefix to the box number. I took a look at the site referenced above by mike-s and one or the other is required. We always used the # sign. From what other posters have said about using no suffix, the PO must not enforce the requirement.
From the site:
"The Delivery Address Line is the standardized address of the private company.
Private companies offering mailbox rental services to individuals and businesses are considered commercial mail receiving agencies (CMRA). Addresses on mail received at a CMRA must adhere to specific requirements in the use of their private mailbox number (PMB). Mail sent to an addressee at a CMRA must include either the PMB identifier or the #, followed by the appropriate private mailbox number. Use of any other identifier is prohibited. Either a three line or four line address format can be used with a CMRA address and the PMB or # identifier, with the exception noted below. Where the CMRA's physical address requires its own secondary address element, the PMB or # address must follow the specific format rules stated below. It is not permissible to combine the secondary address element of the mailing address for the CMRA and the CMRA customer's private box number."