Does adding salt before cooking toughen scrambled eggs?
Per Harold McGee, in On Food and Cooking (page 86 of the 2004 edition), there is no truth to this common assertion.


  Acids and salt do pretty much the same thing to egg proteins.  They get the proteins together sooner, but they don't let them get as close together.  That is, acids and salt make eggs thicken and coagulate at a lower temperature , but actually produce a more tender texture.


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).

McGee concludes:


  So eggs end up more tender when salted, and especially when acidified.


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.