It often improves a wine that's fermented out to add a little sugar (with sorbate) before bottling. Always attracted to the easy way, my source of sugar is Domino's bought at the local grocery store. Typically, I'll make up 3 or 4 tenth test bottles with sugar varying from the original RS to 1.0. After waiting a few days we taste. For some varietals, Riesling or Vignoles, overall taste usually improves. The improvement sometimes varies directly with sugar level and sometimes not, but the taste is better. But for other wines (Friulano, Iona, Pinot Grigio) it is not. Instead, a distinct and unpleasant sucrose taste emerges even at these low levels. Some obvious possibilities: (1) use some other source of sugar; (2) wait longer to taste; (3) some wines just don't improve.
I've looked in some usually helpful places (Pambianchi, Taylor & Vine, Spaziani) for guidance but haven't found much, so I turn here to my ALWAYS helpful winemaking colleagues. Do you generally add a little sugar to your white wines after fermentation? How do you do it? Do you find that the effect varies with the varietal? What have you learned not to do?