I toss mine (I use 3 whites + 1 whole egg per day). There is little point in saving them to use in something else unless you were making that something else anyway.
Buying eggwhites just means the factory is throwing them away instead of me. It isnt less wasteful.
I buy 5 dozen eggs at costco. I dont remember how much but it is WAY cheaper than eggwhites. Besides all of the "100% eggwhites" I have ever seen contain eggwhites plus a bunch of "stabilizers"