Ideally if you're going to be adding large amounts of sugar as a late addition, you want to put it in as primary fermentation starts to slow down, preferably in step amounts (eg not all at once).
The idea here is that sugar (both corn and natural) is easily fermentable and given the choice between dealing with complex malt sugars or super easy to consume regular sugars, the yeasties will immediately head for the path of least work and not deal with the malt sugars.
By putting the late addition sugar in after primary fermentation slows, you're giving the yeasties the simple sugars they prefer after they've chewed through the malt.
Otherwise, if it's a small amount of sugar being added, then a late boil addition (10min or so) it's fine also.