Originally Posted by Indestructible Object
See even doing something as seemingly harmless as say warning Star Fleet about the planet killer from "The Doomsday Machine" thus saving the crew of the Constellation from there original destiny (death) could have dire consequences. For example a member of the now alive crew could become a federation diplomat. Unfortunately during peace talks with the Klingons they accidentally offend the Klingon high chancellor's maternal ancestor... leading to interstellar war. In short you cannot predict what changes might occur as a result of reveling future events to people.
In such instances, Spock Prime would "Take a leap of faith" and trust this would not happen. Too many other events are in flux to invoke the Temporal Prime Directive... If the Vulcan crewed Intrepid were out there, would Spock suggest they die "just in case" ? I think not. Spock Prime would also see no logic in allowing the M-5 disaster to repeat so I think he would keep a close eye on Dr. Daystrom.
We will never know. The Cyclonic events Spock will give warning on and thus we will never have a need to see them on screen since these can be handled in some form or other once recognized. Other things will be in too much flux.
Certain old events like Praxis fall into a grey area. If it meant a chance at peace decades sooner, Spock would try to warn the Klingons... On the otherhand, knowing it to have been the catalyst for the Gorkon Initiative, Spock might not choose to throw away his hole card if a hardline anti-Federation faction held sway in the Empire.