Many cooks have heard that if they salt the water before it comes to a boil, they could ruin their pans. The thinking is that salt's corrosive properties can pit some types of cookware if the grains of salt don't dissolve, but instead settle on the bottom of the pan and stay there for a while before disappearing into the water once it's hot enough.
Will Salt Actually Pit Your Metal Cookware?
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 water that will take an especially long time to heat up. It's also good to consider if you use slower-dissolving salt, such as coarse sea salt. Some minerals in certain sea salts can also affect how quickly it dissolves). Meanwhile, kosher and flaked sea salt dissolve quickly. Table salt falls somewhere in between.
The Case for Adding the Salt Later
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?
Adding salt to water will increase the boiling point, so some argue that salting the water first will make the water take longer to boil. However, for the amount of salt added to water in everyday cooking (water for cooking pasta or potatoes, for example), the increase in the boiling point is so small it's of no practical difference.
You're Good Either Way
If you're still worried about ruining your pans, add the salt later. If you're more concerned about the chance of forgetting to add the salt altogether, salt from the get-go. Either way, most experienced home cooks and plenty of chefs agree that salting water for certain foods can make all the difference between a bland dish and one where everyone wants seconds and thirds.