I would agree with Greg and Rob, especially with how flat todays bows shoot. There are other variables though that can affect shot placement such as how high the stand and how far the shot. I've taken my target out and shot it from stands and in some of the higher ones it seemed like I had to aim shorter than the actuall horizontal distance. Generally though, I aim as normal and for the hunting I do its a non-issue. Here are some links you can read if you like. Seems a little too complicated and not needed for me. Maybe if I was hunting in the mountains or something.
http://www.rayhowell.com/adventures/hunting%20tips/bowhunting%20tips_03.html
http://www.gandermountain.com/hunting/article.asp_Q_id_E_40
http://www.hunting101.com/gunsandbows/bows/bowhunting%20equipment%20and%20skills007.htm
http://www.jesseshunting.com/forums/index.php?showtopic=86491