I have always lived by the rule:  repack the trailer wheel bearings every 12 months or 10,000 miles.  Question:  Is the 10,000 mile a correct standard to go by? 

You certainly aren't doing anything wrong, and you are ensuring reliability. Unless you need to pull the bearings for an annual inspection of the brakes, you could greatly extend your maintenance schedule by using synthetic grease and bearing buddies. An increased schedule of 20k- 30K is not an unreasonable figure while using these products.

Gard