Well, they kind of implied that Nathan meeting Claire was what made him try to stop the bomb. In the future episode, Claire hadn't met Nathan when she met Sylar masquerading as President Nathan. (Of course, that means that my whole notion that the future episode is not an alternate timeline is wrong, as she clearly has met him.) And having Peter save the cheerleader indirectly leads to Nathan meeting Claire.

Of course, what you may be saying is that Hiro visited the future, leaving after Claire had been saved, and arriving after future-Hiro had already visited Peter in the "past". So what actions were taken between the time that Hiro arrived back in our time and the last episode that changed the future. If nothing, and I agree that there doesn't seem to be anything, then that means that Hiro visited a future that had branched off before he left, effectively portaging to a different time stream. So can Hiro visit alternate timelines?

I suppose the one thing you might be able to claim is that Hiro brought Sylar down long enough for Peter to escape with Nathan. Of course, future-Hiro stabbed him, too. And we know he didn't have Claire's healing power to save him.

Hmm. Assuming that future-Hiro did stab him in the same way at the same time, he would have known that it was Peter that was about to explode and that making sure that Sylar was dead would have made no difference. He was incapacitated enough that he couldn't have made a difference in those few minutes anyway. So there may be another bomb event that future-Hiro was trying to prevent. This actually is starting to point to the fact that this might have been a false ending.