Here's what I do when not doing a full repack. I DO use the center spindle zerk to pump slowly about 3 pumps of grease, because it does get to the inner bearing and will fairly efficiently force fresh grease into the bearing cage. If done correctly, it is fairly effective for supplemental lubrication of the inner bearing. But that's where I stop. Filling the hub with grease is just silly.
After that, I use a bearing grease needle to carefully shoot a few pumps into the cage of the outer bearing. I move the needle around the perimeter, inserting it carefully between the rollers. It's not preferred over pulling the hubs and doing a full repack/ inspection, but it works to keep some fresh clean grease in both bearings with minimal effort, time and cost. I also found the first time I pulled hubs to install new bearings and seals that the inner grease hole in the spindle has a sharp burr which can cut a seal lip. I lightly chamfered the edges of the hole to insure I can reinstall the hub without cutting the seal lip. If you want to fill the hub space with grease, have at it. But you'll be doing your bearing and seal life no favors. Good luck.