Base welding is a procedure that is over and above the standard steps of a ski tune and will cost extra. However, if the ski is gouged to point that the wet sander cannot bring it back to flat then it really should be done. Otherwise the unfilled gouge will tend to throw off the tracking of the ski.
Jack and Dan's Tip: Don't ski in the parking lot. Seriously, keep your skiing as high on the mountain as possible during the spring to stay above the melting snow pack. Take off your skis and walk when it gets bare.