There are always many people who 'do' and 'do not do' things for reasons of their own in relation to your question.
I will tell you what I do, as my aim is always to have good effecient, effective equipment that lasts as long as possible and serves me well, without having to be a slave to it.
Some lubricate very often, others not at all. Some with synthetic oils and others swear by mineral based oils.
It is important to give something back and put an oil that finds its way into the bearing and stays a while. SO not too thick, not too thin, you will see it pass into a bearing if you put a drop or two onto the shield ( the part between the inner and outer parts of the bearing.
If the motor is making any kind of noise, the chances are that the bearings have gone past there useful point and will cause problems for you.
In many cases you can take the motor apart and go to your local bearing store and order new, replace them and off you go.
The bearing which always goes first is the one nearest the pinion as this is handling the side forces that come from the pinion and spur gear running together. In my view you may as well change both bearings though, as this will pay dividends in the end.
When you take the motors apart, you will see how you can get oil into the bearing in the future. This may mean taking the motor out again, if it does, then it does, its a part of helicopters.
In order to prolong bearing life people and companies do come up with some great ideas.
For instance. If there is any way you can support the other side of the pinion then this can take a great strain off the front bearing of the motor and make its life much longer.
I fly Logos and they make a part called a 'counter bearing', and this does this fantastically.
I hope this helps...
Plrease feel free to PM
I agree about doing this with all the motors you have.
Esprit Model Flight Team