I was wondering if it makes any difference to the taste, texture, or any other factor.
I tried googling but there are very few resources and some of them are contradicting. If anyone could enlighten me, that would be great.
(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.
I'm not sure how you would salt a sunny side up egg before cooking unless you poked a hole in the shell, but as for during and after I've tried it both ways and I've never noticed much of a difference. All I would say is that when I add it during the salt taste seems to be more distributed throughout the egg than if it is added after.
Scientifically there may be changes made. Salt draws moisture out, which might make a texture difference but none I've actually detected. Salt during may also make a difference in cooking time as a result of the moisture draw, but again none I've detected.
The big difference is really as we say in IT "user choice". If you salt the eggs during cooking you take the salt option away from the person you serve it to, so when I cook eggs for others I never add it as it should be salted to their taste and not mine. When I cook eggs for me I add it during as I like the way it tastes like it it part of the egg itself.
Since salt does draw out moisture, it can lend itself to a tough and chewy egg. That quality is much more pronounced when it's a hard-cooked egg, but as a general rule I salt when serving.
Also, if you pre-salt, the salt dissolves a bit into the liquid and gets dispersed. So I find I need far less salt dressing the egg at the end since there is a lack of dilution. I do like a salted egg and not a salty egg, if you get my meaning.