If it is bad coffee.

When in a restaurant if the coffee is very strong or not so good, my wife will grab the salt shaker and give it a dash. But that is after it is already brewed and in the cup.

Good coffee brewed at the correct temperature and for the correct duration for proper solids extraction should not need anything to correct the bitterness. Also proper water hardness makes a difference in the extraction. Pure water without any minerals may not extract the coffee properly, too hard of water not good also. Or so I've been told. My wife has a discerning taste. It mostly all just tastes like coffee to me. :-)