ideally you stand and draw when the dear's eyes are hidden behind som ekind of cover. you don't always need to have the deer hidden to draw though. the doe i killed saturday was with two other deer and all three were standing in the wide open in front of me 17 yds away. I came ot full draw and none of them knew i was there.
sometimes you just have to take a chance if you don't get the opportunity to stand/draw when the deer is behind cover.