Originally Posted by Mrclasen
However, you should always stir your wort really well before you take your gravity reading with your hydrometer. If you dont stir the wort really well then they hydrometer will give you a gravity reading much lower than what it actually is. This is because the extra gallon of water that you added to the TOP of your fermenting bucket is less dense than the thicker wort that you poured into the bucket at first.
I've had it go the other way - mainly because I use a bottling bucket and end up with a higher gravity - same reason though, the water doesn't mix quickly so I have higher gravity wort on the bottom I'm drawing.
BTW, if left alone over the ferment time, it would all mix but the OG would be off, so it is in general better to mix it.