Do I really need to use a deer decoy?

No. The more time you spend hunting, the more likely it is that a deer will come within shooting range and be sufficiently distracted to allow you to get the shot. However, the deer decoy is more likely to bring a deer in from a distance and keep it distracted and moving around in your immediate area, thus giving you more (and better) opportunities to get the right shot. That being said, it is obviously more important for the archer / bowhunter that must get closer shots.