Worst case scenario is if you ignore the grinding as the rollers disintegrate, before you know it the races will be chewed up, once the races are gone the rotor will be FUBAR, as well as the spindle. Once it gets to that point, which ever wheel lost it's bearing will be sitting at a rather unusual angle, that is if it doesn't get the inclination to run off and leave you behind.
Generally, if they're squeaking, grease 'em. If they're grinding, they are really dry and you'll probably find the rollers badly scored, or falling out of the cage. If that's the case, replace the bearings and races and adjust accordingly.
I generally check/repack my bearings once a year, used to do it more often when I lived in wetter climates (read - flood prone areas). Packing the bearing is a simple matter, I still use the hand pack method. Big blob of grease in the palm of the hand and work the bearing into it until all cavities are filled with grease. Or you could get one of the fancy bearing packing units that pumps the grease into the bearing from a grease gun.
73 280 SEL 4.5