I think the way I will handle time travel is to keep it very tightly managed. My players love to "sight see" as do I. That's part of the reason I'm such a Maztica/Horde/Utter East/Zakhara/Frostfell/Undersea/Kara-Tur and now SJ fan. Time hopping offers so much more of this ability to "explore" that we find so fun, but it could really lead to some dangerous tampering with the time stream.
In the Arcane Age adventure "How the Mighty Are Fallen" I think this was done perfectly. The PCs actions fit right in with the canonical history, but they still got to SEE Karsus bring down an empire. If they are looking to time travel again, I'd definitely try to give them a view of major events, but try to streamline their actions a little more than I usually would.
They're clever, however, and I'm sure they will somehow attempt to stop Szass Tam from growing up, stop the Spellplague, or something similar and I really need to prepare for it.