It can differ from champion to champion, as well as teammate to teammate, but the general rule of thumb is: DON'T DIE. Since you are the fed one, you are carrying the bulk of the team's wealth. If you were to fall, all of a suddenly your team has lost a large amount of power, and is very likely to fall apart in teamfights, tower fights, and objective fights. However, if you were to survive, your team will always have hope defending or even winning fights.
Another piece of advice is to USE YOUR INFLUENCE. Communication and coordination is powerful, and though there are many people who remain silent or even use it to slander their teammates, the chat is essential for getting your team together. A 1/4/2 player may find it difficult for his teammates to take him/her seriously, but a 7/0/4 player will find it much easier to influence the team. In this way, you can be the leader of the team and keep people from doing pointless tasks like hovering around a lost objective or pushing mid when the inhibitor is already down.