It depends. If I just have carry-on, I prefer to be near the front so I can be one of the first to exit. If I have checked bags, it doesn't really matter because the last person off the plane almost always beats the first piece of luggage to the baggage claim area. The back of the plane is usually less crowded, so it's more likely that you'll have an empty seat next to you (or sometimes even a whole empty row). I just don't like being in the very back row, if those seats don't recline.