That’s what reader Daniel wants to know. Daniel, let’s just say that adding salt to egg whites before you whip them is never a good idea. Why not? Well you remember the below post on how egg foams form. Agitation causes the little bunches of proteins in the white to unfold, at which point the individual molecules start to collect around air bubbles and bond with each other. If there’s salt in the mix that bonding process is slowed as the salt dissolves into its component parts — sodium and chloride — and those ions start to adhere to the bonding sites on the protein molecules, preventing the proteins from using those sites to bond with each other. The result is that the foam takes longer to whip up and is less stable when it finally does form.
The logical question then is: well, what if I added the salt later, as I would do with an acid to help keep the proteins from over-bonding and coagulating? Some bakers do that, however for my money acid and copper are more reliable — and less potentially destructive — foam protectors.