To add to your "until there's an issue" part of the question, my bearings ('71 big bearing) had not failed, nor were they making any noise or giving me any indication of looming failure. However, as a good practice I checked them out when I simply had the brake drums off while doing a service.
I could move the axles up and down slightly and the bearings sounded dry to me. Not knowing anything but what bearings should sound and feel like, I decided to change them. And glad I did!
They would probably have lasted another 15k miles but they were still toast at 90k in my case. And the first thing I noticed when I put the new ones in was that the Bronco rolled much easier.
I had always put it's resistance to motion off on it being a heavy truck with big tires with more rolling resistance. But it was in fact the wheel bearings having been worn out before their time.
So the answer I gave previously still stands for most, but in my case it was "until there was an issue" that I found early-ish and was able to fix before any problems arose.
Unfortunately I did not notice any better gas mileage!
But I slept better after that too.

;D
Paul