Do I add salt to my sunny side up eggs before, during, or after cooking?
(1)  Taste.  When you salt your eggs DOES affect taste because it affects the way your tongue comes into contact with the salt.  If you salt your eggs before or during cooking, some of all of the salt dissolves in the water that is in the eggs (raw eggs, overall, are roughly 75% water) and is dispersed over the surface of your eggs while they cook. When you salt your eggs after cooking, crystals of salt remain on the egg and, when these crystals come into contact with your tongue directly, they produce a brighter salty taste.  So basically, the way your tongue registers saltiness depends on concentration, not just amount.  Think about how differently a pinch of salt tastes directly on the tongue as opposed to the way that same pinch would taste if dissolved in a drop of water.

So...the tongue registers a brighter salt taste when it comes in contact with salt directly.  That doesn't mean that salting eggs after cooking is better - it means that it affects perception of salt differently.  Go with whichever you prefer personally.

(2) Texture.  Whenever I have a question about food science, I consult Harold McGee's book "On Food and Cooking, The Science and Lore of the Kitchen."  He states unequivocally that salt DOES NOT toughen eggs.  According to McGee, the only egg preparation that is harmed by the addition of salt is and egg white foam (whipped egg whites).

So from the standpoint of texture, salt whenever you feel called to do so.

Controlling the heat when you fry an egg is the single more important factor in determining the texture of your cooked eggs.