This thread should generate some activity, due to the many opinions on the subject, and the CYA recommendation of the manufacturers.
My opinion...bearing service is either done way too often, or just completely ignored, until there is a failure. I do the bearings very soon after purchase, when new, and right away if trailer is purchased used.
Once I have inspected the bearings, and packed them well, with quality grease, they are good for 4-5 years, in my use. The seals should be replaced any time they are removed obviously.
I have used the zerks also, and no issue with the seals leaking grease onto the brakes. Follow the manufacturers recommendation on the proper procedure for zerk use. A tip...I tow the trailer to warm the bearings/existing grease first, before using the grease gun, that I lay in the sun to warm/soften the grease. I add enough grease, to see fresh coming out the front bearing.
Jerry