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I never put salt into the water when I boil it to make pasta even though cooking directions always suggest I do that and the pasta comes out fine. How necessary it is?
Salt is not necessary, it's just a flavour thing, a common personal preference
I suspect you'll get a saltier taste by putting it in the water, than putting it in the sauce for the same volume of salt
Hot water is not necessary too, you can prepare dried pasta with cold water. But it doesn't taste right to me, so does salt less pasta :-)