I learned to add a bit of salt to all sweet recipes when I was at the Food and Wine Classic in Aspen.  We sat down and were given two servings of the same desserts, one of which had a pinch of salt, the other did not. We were not told what the difference was at the time just instructed to taste them both and then write down which we preferred, and why. We all preferred the one that had a pinch of salt. It tasted sweeter and more rounded than the other dessert. Since then I always add a pinch of salt.