The thing is, most bearings aren’t built specifically for bike use, bike stuff almost always uses industry standard bearings which are generally built for a) higher speed and b) in this size, usually inside something and not very exposed to dirt, weather etc. So low fill is the way to go, and also the grease used is picked to work with that.
For us, that doesn’t apply, our fastest bearings are still moving slowly, hundreds of rpm, some don’t even spin and rotate through only a few degrees. And often the rubber seal is basically the only protection as we splash through puddles. So a bigger fill is just better, it keeps dirt and water out with trivial downsides. The challenge is that it’s possible to damage the seal, if you’re clumsy or use inappropriate tools, but as long as you do it well it is simply better. And it’s easy to test that, for that matter, you can do one side of a wheel but not the other and see what happens. Waterproof greases tend to be much stickier which is unsuitable for a fast bearing but can work great for us, there’s no need to clean out the original factory grease but if you’re really obsessive then you can, just be very sure that you’re getting new grease thoroughly into it- this isn’t so esay with caged bearings.
In the end, none of it’s really transformative as long as you use quality bearings in the first place. But especially in the winter months if you want to get the maximum life out of hte parts and reduce future maintenance/replacement/failure, then greasing is probably worth it. Bikes that sit in a cold garage getting used less can also benefit more.
As far as servicing goes, once a sealed bearing is dirty it’s very hard to clean, pretty impractical in situ, and taking them out usually means hammering out hte central race which will often damage the bearing enough that it’s not smart to refit. And once it’s rough it’s knackered, you can extend its life but you can’t fix it. So for most people once you have a bearing in the frame it doesn’t get serviced, it just gets replaced.
I’ll happily repack a worn bearing just to keep it going a little longer especially when it’s inconvenient to replace, ie, I am being lazy or the timing’s bad. And you can run a worn bearing for a very long time, situationally- frame bearings tend to get crunchy and sticky and can sieze completely and damage parts, but wheel bearings tend to loosen up. The big plus of sealed bearings is just that you have to try rreally hard to ruin the wheel or bike or whatever, it can be done but it’s not like cup and cone or some bushings where even a little neglect or not noticing a problem can be seriously bad.
