I think if you have the knowledge of good/bad engages then you should try and lead your team.
If you cannot manage that then I think you just need to ask yourself what you living or dying provides your team. If 4 team members go in for a fight then you HAVE to join because even if you live you still lose just as many towers or whatever because you cannot defend by yourself. If you go in maybe you can get another kill or summoner or something. Maybe you see that your team is going to engage and you know you cant win the 5v5. Then just go split. Basically you just are trying to adapt to your team and stay the most useful.
Say you know someone is going to chase a guy across the map. Then you can expect the other team to collapse on him. So then you should conclude to go and push a lane and get a tower. Maybe now they collapse on you and you die, but now your teammate got the kill with his life. That is sorta a risky play, but it gives the best chance of winning. Do you sorta see what I am saying? Like the alternative is that you chill all safe and he solo dies or dies with teammates that tried to help and then you just get pushed and dove on. And what if no one comes for you? You get towers out of their deaths.
This is difficult to answer because this game is so situational and never the same. It really depends on team comp and who you are playing.