I have been told by two different cooks who's opinions I trust that adding salt to a beaten egg mixture before cooking will dry out and/or toughen the eggs, but also that adding salt at this stage seasons the eggs, and does not affect texture. I can't tell the difference in texture, but my palate isn't the most refined. I imagine the salted eggs taste better, but it's a slight rather than dramatic difference, if there is a difference. Can anyone with a golden tongue or superior scientific method tell me whether to season before or after?
Per Harold McGee, in On Food and Cooking (page 86 of the 2004 edition), there is no truth to this common assertion.
McGee explains that the salt separates into ions, which cluster around the charged regions of the egg proteins, allowing them to get closer before being denatured because they don't repel each other. This causes them to bond together earlier, before they become unfurled and could form tighter (and thus tougher networks).
So salt them when you beat them, or in the pan, in practice it will make little difference compared to many other factors, such as the temperature and technique with which you cook them.