After rotating and flipping your wheels, the imprinted sides should all face the opposite direction. Bearings Prevention ins the first step in bearing maintenance. Prevention means skating clear of puddles and sand. Water and sand are the archenemies of bearings. If your bearings don't spin freely or make grinding noises, it's time to service or replace them.
Serviceable bearings have removable shields and c-clips.
Non serviceable bearings are like disposable cameras. Once they're used up it's better to buy new ones. Non serviceable bearings have crimped metal shields that are difficult to remove. If you have non serviceable bearings that are old and cranky, I recommend replacing them with serviceable bearings. Remove bearings from skates and wheels using a skate tool and bearing pusher.