Any zerk fittings on an axle or anything like that should only be used to completely replacing grease. Once you pack wheel bearings and install your hub, you do not need to add grease. If you need grease, that you have a problem. You should be able to go 10,000 miles easily before needing to repack bearings. The only time that’s not the case is if you start getting water in the hub. Knock your dust covers off once a year and check for water ingestion. look at the backside of your hubs every month and see if you have grease slinging.