I figure that it should be used, rather than tossed. Last time, I made a three gallon batch of mozz and just kept out a gallon of it for the stretching. I added salt to the whey (only about a half cup, I believe), rather than try to salt the curds.
When I was done stretching my mozz, I added the rest of the whey back in and made ricotta. I think the yield was better because of the additional whey that was expelled from the mozz went back into the pot, rather than being wasted in hot water. Plus, the ricotta didn't require any additional salt - the amount was perfectly balanced in the ricotta.
The used up whey was fed to my chickens. They've been producing eggs really well this winter, so this cheesemaking benefits the entire farm. I love it!