Before I dis-assemble things to repack the bearing, I use a punch to mark 12oclock on the retaining nut. When all is complete, I put that nut back to the same position. There should be no reason for the nut to be tighter or looser after a bearing re-pack. The wheel should spin freely, with just a hint of wobble.
My test for too tight of a bearing is if I can't hold on to the hub at the first stop/50 miles, then it's too tight. Been there, done that with the first pop-up - had to repack bearings with fresh grease at the truck stop about 500 miles from home. Having brakes on the trailer will also generate heat, so you would minimize their use prior to checking the hub temp.
I have seen more bad bearings from non-use (more motorcycles than trailers). If the grease hardens and moisture builds up on the bearing surfaces, those rusty bearings need replacing. As others have stated, if the grease looks like honey - its full of moisture. A good inspection should be done on the cleaned bearings and races prior to repack.
Lastly, I would be worried if the wheel was looser after 100 miles than at the repack - what the heck moved to make it looser? If you don't mark the nut first, standard practice after a repack is to tighten up the nut, then loosen it until you get the little bit of wobble. Do this a second time to see if the rotation of the nut is in the same place when the wheel starts to wobble just to be sure. Sometimes with castle nuts, it may seem that you have a choice between too tight and too loose. Choose loose. Those of you afraid of a hint of wobble have never tried to induce wobble on a spinning bicycle wheel!
