It seems to be conventional wisdom to add a pinch of salt before beating the eggs.
However, this website gives a scientific explanation of why it's a very bad idea.
Could you tell me who is correct?
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.
Edit: Harold McGee's book "On Food and Cooking, The Science and Lore of the Kitchen" states that egg white foam (whipped egg whites) is harmed by the addition of salt. Source: this question.