Best moment to put salt in spaghetti?

Salt should be put before putting the spaghetti (or any other type of pasta for that matters) in the water.

For 200g spaghetti (2 people) count ~2-3 liters of water and 20-30g rock salt. You can reduce the amount of salt if the sauce you are using is already quite salty.

As a note to your point 2, the salt INCREASES the boiling point of water (a process known as boiling point elevation)

However, the increase in boiling point when adding 20g of salt to 2l of water is practically insignificant.

Salt doesn't lower the boiling point of water, it elevates it. Even so, the amount of salt you add to pasta water (10g/litre is a good guide) will barely make a difference. You need to add nearly 6 times that amount of salt to a litre of water to raise its boiling point by 0.5°C.

As throwing things into boiling water can result in splashing, I suggest adding it to the water before you bring it to a boil.

I'm going to argue for adding the salt after a boil, but before adding the pasta ... because I have stainless steel pots.

If you add salt to cold water, it won't disolve and disperse quickly. This results in the salt falling to the bottom of the pot, then slowly disolving there but not mixing. This increased concentration of salt can end up causing pitting.

Instead, I bring the water to a boil, toss in the salt, then bring it back to a boil (mostly because the lid was off, not because I significantly moved the boiling point), add the pasta, stir it 'til it's all fitting in the water and not clumping together, put the lid back on, bring back to a boil, then reduce the heat (as we only need to maintain a boil we need less energy than trying to elevate the temperature; the less rapid boiling also reduces the odds of the starchy foam boiling all over the place).

KISS ( Keep It Simple Stupid, words I live by. ), add the salt to the pasta(almost all pastas will benefit from salt) water anytime after the water is added to the pot and before the pasta is added. Just please...add the salt.

If you are concerned about corroding your stainless steal pans...(shrugging) find something else to worry about.

There is a reason that stainless is also called CRES(corrosion resistant) steel. You can cause stainless to actually corrode but it's not easy, your more likely to cause discoloration that can be cleaned with a product like Barkeepers Friend.

Some people believe that you should not add the salt to the pot until the water is boiling because, allegedly:

• It can cause damage to your pot, inducing pitting in stainless steel. We could not find strong confirming evidence of this (see this question), especially at culinary concentrations and temperatures.

• It can raise the temperature that the water must reach to boil. This is true, but trivially so: the temperature difference is on the order of 0.3 degree F. It makes no practical difference in cooking.

The real limit is that you want it dissolved and distributed throughout the water when you add the pasta to begin cooking. I suspect an actual study would show that with strong boiling and the convection it creates, adding the salt with the pasta would probably be equally effective in the sense that it would be distributed very rapidly, but I am not aware of any research here, and it is just as easy to add it earlier.

Since there is no strong credible reason to delay adding the salt, I add it along with the water personally, since it is easier not to forget.

Note: I wrote this answer for a new question, before I saw the duplicate.