Hiya!
Please, no. This goes for @
pming too. This is a great idea, except the "roll for it" part. I can't count the number of times that a player has had a great idea, and was super excited about it, just to see it quashed by the DM saying, "okay, roll," and getting a low roll. Sometimes it's not even a low roll that spoils the fun, it's an average roll that spoils the fun!
I'm with Horwath here. Time travel is a nightmare for anyone who stops and actually thinks about it. For everyone else, it's a barrel of monkeys. So I'll one-up him: I wouldn't touch it with a 12-foot-pole.

So rather than use that players great idea you'd rather just say "Nope" to it? If you think it's a great idea, why wouldn't you want to take the ball and run with it to see where it goes? Seems...odd...
Anyway, my reasoning for the random roll is to removed the inevitable long, drawn out, and probably heated debate with the players.
DM: You get back and the world is run by nazi's!
Player: What?! Why? We took out Hitler fer crying out loud!
DM: Yes, but Herr Schtoofer took over and was just as bad, but didn't attack Russia until last...
Player: But we gave all the rocket info to everyone, including Russia...
DM: Yeah, well...hmmm...in that case...you get back and the world is now a post-apocalyptic wasteland!
Player: WTF?! Because everyone had rockets? That's called mutually assured destruction. Why would *everyone* get in on it? That makes no sense...
DM: Because, well, crazy people did it.
Player: So no matter what we did, you still just 'decided' to mess up the world for us? We couldn't have made it better? ...yeah...real fair, Mr.DM... *fume*
That is why I suggested a random roll. It takes the "absolute DM fiat" out of the equation. Unless the player characters never go back to their own time line, there is no feasible way to "play out" likely scenarios and outcomes. Especially when we are dealing with years, decades, centuries or even millenia.
Somebody mentioned the butterfly effect near the beginning of this thread. That theory would lead to the most interesting outcomes, IMHO, so I'd use that. And as I just said...a near infinite amount of variables for a DM to consider to come up with a "likely" outcome for any PC time-meddling. Because of that...random roll to determine horrible, bad, neutral, good, or awesome result of that meddling. Even/Odd, dice vs. dice, number spread, whatever the mechanic, a random roll is the only way to be both fair and cover all the variables. IMHO, of course.
I do agree with the "not with a 12-foot pole!" though...I'd rather just not get into time-travel in my D&D game (at least not in any meaningful way). But if the players really wanted to...random outcome. And yes, I'd let the players know this mechanic before hand...otherwise it's kind of a dick move.
^_^
Paul L. Ming