The rules regarding the use of time travel aren't all that hard. Just because something happens doesn't justify the use of time travel to fix it. However,
if an incident happens due to some sort of temporal interference, then it is justfied to use time travel to resolve the situation.
Nero time travelled, intentionally or not, and caused all kinds of havoc. Spock is more than justfied in using time travel to prevent that situation from happening. Written properly, Spock would know that, and would immediately begin his calculations for the proper time jumps.