The policy says A+ gets priority:

Free same-day standby is not available at Southwest® kiosks. Please see a Customer Service Agent at the airport for this benefit. Free same-day standby will be provided for A-List and A-List Preferred Members traveling prior to the original scheduled departure, between the same city pairs, on the original date of travel, where a seat is available. On flights that do not meet these qualifications, A-List and A-List Preferred Members will receive priority standby and will be required to pay the difference in fare if a seat becomes available. A-List Preferred Members will be prioritized ahead of A-List Members.