As other said, it's not a reg. I'm not even sure I'd necessarily classify it as a recommendation.

What I think of it as is information: a normally operating DG will precess about 3 degrees over 15 minutes. If it precesses substantially more than that, the pilot is increasing the risk of having it go inop in flight.

The regulatory issue, I guess, comes into play if you take off into the soup with a DG that precesses a lot and end up having a problem as a result.