BAck about 32 years ago, when I was connected to a Porsche dealer, we suddenly got the rather surprising directive from the factory that we should stop repacking wheel bearings as part of the regular service (every 12,000 perhaps?) unless there were signs of escaping grease to indicate a failing seal, because the engineers had concluded that far more wheel bearings were going bad because the people who repacked them got dirt inside than for any other reason. The bottom line: if you feel your bearings are a bit too loose, adjust carefully and cleanly; if you detect a leak, do a complete service, but otherwise, keep them buttoned up and just watch closely for escaping grease or signs of slack.