Adding salt to the water you cook your pasta in makes a huge difference in the flavor of the finished dish. Salt brings out the flavor of dry pasta, which is generally made from only flour and water. Since pasta absorbs water as it cooks, it absorbs the salt from salted water and is seasoned throughout in a way that can’t be replicated by salting after cooking. Compared side by side, pasta cooked in unsalted water tastes flat. In order to taste a difference, though, you must use plenty of salt; allegedly there’s an Italian saying that your pasta water should be “salty like the sea,” meaning that you can taste the salt in the water.
You can add salt to the water at the beginning or when it comes to a boil. Salted water boils at a higher temperature than unsalted water, but the difference is so small as to be unnoticeable. The only real issue with adding salt before you heat the water is that it may pit some cookware. Adding salt after the water comes to a boil avoids this potential problem, but the water will take longer to return to a rolling boil, since it must rise to a higher temperature.