Spoilers!: backtracking to the start

[Hakoto]

Trying to go for a true pacifist run and I heard on the web somewhere you can go back to the very start of the game at some point and get some extra dialogue from toriel. At some point she disappears from the file start screen during certain hours on your system clock which tells you to go back. Is this true and when can you trigger it?

[Nixitur]

I'm pretty sure that once you leave the Ruins, the door is closed until the ending.
You can, however, backtrack through the Ruins between sparing Toriel and leaving the Ruins which gives you a tiny bit of dialogue from her. It's not that important, though.

[Senator Jim Death]

At the end of a true pacifist run, you can go back to the first screen of the game for some extra dialog. It's worth it. However, I've heard that you can't get it once you've quit the game after the last boss, so before you do that or even enter that last ruins door (the game will tell you that you're about to really end the adventure) you should go back to the first screen.

Everyone pretty much has something new to say, so it's a worthwhile trip.

I have heard nothing about getting back into the ruins at any point other than this ending. It's possible your source was confused or misleading.

Now, a totally separate topic would be an investigation of the file start screen. I get the feeling that it's got some secrets that nobody's been able to notice yet. It does change as you befriend characters, and I imagine as you murder them on a genocide run, but I don't know any of the details.

[fluffy]

On a genocide run it gets emptier and emptier, just like the towns.

[Sv443]

I actually backtracked to the beginning of the game after getting the true pacifist ending, before you go to the surface, where Toriel finds you and on the way there's really nice dialogue but after reaching the beginning you meet Asriel again, where he is watering the flower bed and you can spam talk to get more amazing dialogue