I take mine to Saunders in st Jacobs. They told me to not use geese nipples  no more. They said they are only good for adding some extra  greese when clean bearings are installed. After that they shouldn't be used at all. They just cleaned them up re packed them. I do it every 5 years because if you don't can seize burn tire up and wreck axle and you are stuck on side of road. Happened to my dad that thought they be good forever. A few hundred spent on service will prevent some headaches when you need it. My trailer 14 years old repacked  bearings twice never replaced yet. I do a few thousand kms a year typically 

You absolutely can put too much grease in a bearing. You only want to pack the bearing to at most 70 percent. If you pack it entirely, it can damage the seal, cause higher friction and overheat it. 

thank you @CAT RIDER.   That makes perfect sense.     I have a 10 yr old  6x10" utility trailer with 15" wheels that were repacked 5yrs ago with synthetic grease and never touched since.    I've always looked at the seal for leakage and figured since it all looks ok there is nothing to be concerned about.   That single lip seal will only keep grease in and not keep dirt out like a double lip seal. Think I'll just purchase and entire trailer wheel bearing and seal kit and make all fresh rather than bothering to clean the old ones

This is my thoughts only,  trailer tires are smaller in diameter and travel at a greater speed  or reveloutions per minute than a tire on your truck. The seals are not usually as good as what is used in automotive applications. Therefore more speed requires more maintenace.  they are more septable to failure on the inner bearing due to moisture getting in from the back side. It  is way easier to do prevetative maintenace  than deal with a bearing failure on the side of the highway. I use only brand name bearings not made offshore. Upgrade the seals to a double lip type with spring. If the spindle gets pitted where the seal rides I have used a speedy sleeve with success. Depending on the axle, some are drilled if you use a bearing buddy to deliver grease between the bearings so the inside bearing gets grease. Other bearing buddies only grease the outer bearing. I like to wash clean and check bearings yearly or max every other year depending on milage. Its part of my fall maintence plan for sled and trailer. For a bit of time and a few dollars its not worth chancing and ruining a trip and I never want to be that guy that has a wheel off on the highway!

Going to revive this old thread, with my question:   Why do you need to clean out and repack trailer bearings, when car wheel bearings will go 100,000Kms without any attention?