Your ideal positioning as ADC varies from game to game depending on numerous factors such as the enemy comp, your own comp, your champion, how fed you are, how fed the enemies are, WHO'S fed, how much your teammates are aware of their need to peel, and how aware the enemies are of their need to kill you first. A champion like Ezreal with a good escape can naturally be more cocky than Jinx as far as positioning goes, and having a team comp like Janna/Lulu/Kayle/Lee Sin/whoever the F you are as ADC gives you a lot more lenience than a Sona/Nidalee/Akali/Fiora/random ADC team comp, for instance.
Essentially, you want to be in a position where you can dish out constant damage to maximize your DPS, but also where it is hard to get you with high-burst champions and LOLUDEAD skillshots, because even if you are ahead, you are skill squishy, and unless Fizz or Zed lost his lane in a rather spectacular way, they will still 1v1 you with ease if given the chance (Fizz especially is a bitch to peel for your support because LOLCANTTARGETME). Same goes for eating a Morgana bind or Blitzcrank pull to the face; it is quick and decisive death. It doesn't matter if you're 15 kills ahead when Blitzcrank pulls you into his whole team, or when you are snared for 3 seconds: you will die, and late game you will be dead long enough that it can mean game over for your team.
You said "As late game rolls around, when we are very much ahead, with virtually no risk of losing". This statement does not work. If you reached late game, no matter how ahead your team is, the enemy still has a chance to win. If you were winning this hard, there wouldn't be a late game. We don't call it throwing for no reason. When late game hits is when you want to be the most careful because the people who want you dead on the enemy team are likely full build (in any case, they have enough dmg to solo you), the enemy ADC whom you presumably won lane against got time to catch up, and death timers are longer than ever: if you get caught and die first in a fight, you're gone for at least 40 seconds. 40+ seconds without your ADC is a very long time during which the enemy can easily Baron, or push down a lane: your team will be at a disadvantage because you are not there. Your first responsibility as marksman is to NOT DIE. Dead ADC dish out 0 DPS. You might need time to kill a bruiser or a tank, and you might need to disengage from a threat before actually joining the fight with the rest of your team, but you're still doing more damage than if you were dead.
I understand the desire to be aggressive, and the frustration when your teammates who are supposed to be engaging fights don't seem to be doing anything in that regard, but while we still tend to refer to these champions as CARRIES, marksmen need their team. As a support main, nothing makes me more nervous than an overly aggressive ADC, because I never know when they're going to get caught and deleted too fast for me to do anything about it.
Darkshade24 
