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).