Even if you have EZ-Lube or Ultru-Lube greaseable axles, the manufacturers state that you should remove, inspect and repack the bearings annually. I asked the head tech. at our dealership about this, and they were adamant that you should never rely on just pumping grease into the axles yourself and forgoing annual re & re. For one thing, the bearing quality is not all that great and a visual inspection allows you to see exactly what's going on.
It seems like pumping grease in is a good idea in a new trailer because the factories don't always put enough in. Also a good idea if on a long trip in a season. Doing an annual brake inspection is a good idea and a good time to do the bearings as well and for the $250 or so cost, cheap insurance.
'Course, there will be some that they have never removed and repacked their bearings and are fine 20 years later....