Who in their right mind would put salt in a cup of coffee? Someone stuck with a seriously substandard cup of coffee, that’s who. “I will never suffer through another bad cup of coffee at a fast food joint, restaurant, gas station, etc., as long as I have salt to add to the cup,” says rworange.
A very judicious sprinkle of salt (just a few grains, really) indeed takes the edge off a bad cup of coffee, taking away the sharp bite and making it mellow and smoother. It won’t become great coffee, but it will be decent. However, adding salt to decent, non-bitter coffee can make it worse, making the subtle defects more obvious. (No word on salt’s effect on really good coffee.)
Some additional findings from rworange: Less is more–you don’t want salty-tasting coffee. Add only the tiniest bit and taste; when it has mellowed the brew, stop. And it makes no difference in flavor whether salt is added to coffee grounds before brewing (1/8 tsp. is plenty for a 10-cup drip coffee maker) or to a finished cup.