Originally Posted by YooperBrew
What was your OG and FG? If they are in line, I'd leave it.
Two very good questions in one by Yooper.
If you're OG is right where you want it or it needs to be, don't add any water, because no one likes watered down beer (that's why we started homebrewing and drinking craft beer right?).
I would assume, and don't have anything scientific to back it up, but it may make the yeast go dormant before they finish fermentation, as they will recognize that there isn't as much "food" available for them in the diluted beer.
If your FG is a high, then it would be ok to add water at bottling/kegging time, as the water addition will help to lower the FG. But I would add it to the bottling bucket or keg, as adding it directly to the fermenter could stir up some trub before siphoning it off.