Does a salt water pool need to be drained annually?

In a Word, no. In fact, if the pool is vinyl or fiberglass (or you live in an area with a high water table) draining ANY pool is not a good idea. A drained pool can pop out of the ground or the liner can be ruined unless special precautions are taken.

IF you live in an area that gets hard freezes a pool needs to be winterized but that is not the same as draining it. Since salt pools do not use stabilized chlorine then they do not become overstabilzed. The only reason water partial replacement might be needed is if the calcium hardness is very high and this often involves trucking in softer water is the supply water is very hard. (however, in areas with high calcium hardness, such as Arizona, there are companies that can 'process' the water in the pool to remover the excess calcium so water replacement might not be neceessary)