I think it is a good idea to do a full bearing cleaning and packing any time you purchase a used trailer that you do not have a good service history of.
I purchased a lightly used TT but the wheels had been under water at some point and some of that water made its way into the hubs. Some of the bearings were not spinning freely due to the contaminated grease and this resulted in some pitting of the races. I ended up replacing all of the seals, races and bearings on two of the four wheels and the other two just needed a cleaning and repacking. Once you have done one wheel, you have a good idea of the process and if you don't need to replace any races, you could do a wheel in about 20 minutes. It takes a bit longer if you are trying to salvage the existing inner seals. Otherwise, you can beat them out pretty quickly and replace with new. Just make sure to use a good waterproof bearing grease. Also, if you do need to replace any races, save one of the old ones and cut a radial slit in it to use as a race driver - works great.
I also have the EZ-Lube spindles and even though you are supposed to be able to fill the entire cavity with grease, I'm still a bit wary about an inner seal failure that would result in a lot of grease being thrown into the brakes and that would be bad. I do take some long trips but I think you can go a couple seasons between a full teardown and I tend to hit each spindle with a couple pumps from the grease gun a couple times a season. I actually enjoy repacking bearings and I would do it every year if I had the time but in most cases, I don't think it is really necessary to do annually. If you were going to have your hubs overhauled by a shop, a yearly trip could get pricy.