You can do it now or before bottling, either one. I have done it both ways. Sometimes when I want a selection I backsweeten at bottling time. I take out several 1/2 cups of wine and progressively sweeten them to see which I like and to see how much sweetener to put in each bottle. Sometimes I use white sugar and sometimes a mixture of sugar and concentrate. It is a little more work this way but you can leave some dry, some semi sweet and some sweet if that's what your looking for.
On the other hand, if you know you want it a certain sweetness, it is easier to just rack it onto sugar or concentrate and either let it age more or bottle it. Oh, and I do like to make a thick simple syrup as it mixes with the wine.