Many cooks have heard that if they salt the water before it comes to a boil, they could ruin their pans. It is true that prolonged contact with salt can cause pitting on stainless steel surfaces, and salt placed in cold water takes longer to dissolve. So yes, theoretically, the salt will sit on the surface longer, and this has the potential to affect your pan’s surface. This could be a factor if you have a really big pot of waterr that will take an especially long time to heat up, or if you use slower dissolving salt, such as large coarse sea salt (some minerals in certain sea salts can also affect how quickly it dissolves). Kosher and flaked sea salt dissolve quickly, by the way.
But for most of us using a pinch of salt in a 4-5 quart pot, it is unlikely to make a difference. On the other hand, since there is no advantage to adding the salt earlier, why not play it safe and add your salt once the water is good and hot?