Front? when you're greeted by grease leaking out of the rear of the backing plates/drums (not terribly common), if you're careful doing your brakes, you can usually re-use them and keep on kicking.
Rear? depends, but on the swing axle cars, you can start to see transaxle oil leaking out the drums while the Independent Rear cars are like the front ones, they will actually leak out grease, not usually common. Most of the ones I replace get done with the bearings ( they sometimes disintegrate when you try to pull them.) One little note: If you do the rear seals, go to one of the places that deals with the air-cooled stuff as you can get them in a kit that will include the large "oil seal", an O-ring, paper gasket, washer and cotter pin. Be sure to clean the coverplate really good on the swing-axle cars as they have a small drain hole in them to let out any overflow of gear oil. If you are replacing the front ones, they are different for drum/disc set-ups.