I never repacked bearings on my previous trailer (1998 model we bought in 2008, previous owner packed every year) the 4 years we had it. I haven't done our current trailer yet (2013 model), but plan on it this year. We have not been going very far and not very often yet. We had big plans to be out 3 days per week all summer, but life changes...
With all that said, I have checked the temps by hand on each wheel and bought a infrared temp gun last year so I can detect a problem starting. A higher temp on one wheel might indicate a pending bearing or brake problem, something I can take a look at.