You're supposed to clean the bearings???
Just kidding. Actually, I take the seals out of my bearings, flush out all the grease, and add a few drops of bearing oil. Some people have said "you'll ruin your bearings" and "they'll only last a race or two". Well, I have had the same set in there since March, and I race pretty much every weekend. I have only had the bearings out twice. I flushed them out an re-oiled them and they have been good to go. I race on a permanent ashpalt track that is swept and blown off before racing. This may not work on dirty tracks or unswept carpet tracks.
But, it does make a noticible improvement in the way the car runs. I don't have any actual lap time numbers to give you, but it was noticibly faster. Same motor/gearing/batteries/esc as before I did it.