The thought for adding at the end of fermentation is to make the yeast eat the maltose first (harder) then move to glucose (easier)...don't eat dessert first and spoil dinner sort of thing.
The truth is, many add sugar at the end of the boil (myself included) and have no problem with fermentation/attenuation. I find the boil to be much easier.
If you add during fermentation, it would be a good idea to dissolve it first in some water. Helps it mix quicker...some guys have had a layer of undissolved sugar on the bottom of their carboys before. Oh, and you can use table sugar too, it's cheaper than corn sugar and does the same thing.