I used to work for a company that made these types of cabinets (Protectoseal). We competed with Just-Rite, Eagle, etc. I have two 45 gallon and two 60 gallon cabinets from those days. If you decide to use the vent, make sure it still has the flash arrestor in place, otherwise it defeats the purpose of the vent. If all you plan on storing in the cabinet is cleaners, spray paint, etc., just unscrew the vent plug 3/4 turn or so and it should vent any pressure build-up fine. Make sure you ground the cabinet properly, though. I can't tell you how many industrial plants I visit that have flammables in these types of cabinets, yet do not have them grounded to eliminate static electricity spark hazards.
Neal.