I sometimes still wonder under which circumstances it is considered ok to muck with the past.
In the futures of Endgame and Timeless the authorities didn't want the timeline to be messed up, even if it would lead to a 'better' future (Voyager not crashing on that ice planet, Voyager returning sooner to the AQ than those 23 years). In Trials and Tribble-ations they try to disturb the timeline as little as possible.
Then, there is Yesterday's Enterprise. Picard sent the ent-C back, based merely on a vague feeling by Guinan that things were 'supposed' to be different. Would official Starfleet policy have been to not do that, even if their situation was becoming desparate?
Of course, from our viewpoint, Picard does the right thing, but as far as he knows, his reality is the reality.