Some things about altering the timeline:
1) Spock can't alter it too much to one party's advantage or others would do it too. I.e. Sure he can warn the Federation about the destruction of the Deneb colony, but he'd also have to warn the Klingons about the loss of their Ch'whatsit colony by the flying killer brains of Jabnab IV.
2) He can't give the Feds too much in the way of weapons knowledge or the Klingons would have a fit and send their own emissaries back in time or creating alternate timelines/dimensions - possibly ones that would come to the aid of their brothers in this one.
3) If he told everyone about the loss of the E-C saving Narendra III, would the Klingons believe him securing peace earlier; would pro-war parties in their government make a war happen sooner not wanting peace with the Feds; would the Romulans strike early?
4) Even giving medical knowledge to everybody he could remember, what if the Tzenkethi got all pissy their diseases weren't remembered and started a war over it. Or if the Sheliak found the spreading such knowledge as dangerous as the proliferation of WMD and started a war over that?
All that said, I don't know I wouldn't spill everything I knew were I in the position to do so, hoping that the info I spread would be used for the best. Still, for the entertainment franchise that is Star Trek, it can't be changed so much that it's unrecognizable to its fans/profitable.