In Carnival of Monsters, the Doctor says the SS Bernice vanished in 1926. Fact. Later, Jo asks if the ship has returned home and the Doctor says it has. However, each of the people on board have no life on Earth after 1926. To put all of them back into the web of time will create massive ripples. So the Bernice did not reach Bombay. What happened to the ship once it returned to Earth is anyone's guess. Perhaps the freak tidal wave did get it. Or maybe the Eternals happened along and took the crew for their race (see Enlightenment).
In The Aztecs the Doctor claims that history is unchangeable, or at least that the effects of a time traveller's involvement is in itself part of history. This may be because the Doctor is new to the business of time travel, is worried about the Time Lords catching up with him upon seeing any obvious examples of time 'editing', and does not know how much he can trust his human companions.
In The Reign of Terror the Doctor states that Napoleon can't be assassinated because they know it didn't happen (whereas if this were true he wouldn't need to warn Barbara in The Aztecs). In The Romans the Doctor is rather pleased that he gave Nero the idea to burn Rome (but only after he's made a show of non-intervention for Vicki's sake).
From this point onwards the Doctor's attitude to history is that it is fragile, but he trusts his companions not to meddle. Certain events seem less important, historically, than others (for example The Gunfighters, where the Doctor seems actively interested in changing history).
In The Time Meddler, Stephen and Vicki speculate that if the Monk succeeds in changing 'history' then their knowledge of historical facts will, in an instant, change. However, in The War Games, despite placing Jamie and Zoe back in their own times, the Time Lords have to wipe their memories themselves. In The Time Monster, the farmer's memory of the wartime V1 crash isn't changed by the V1 crashing contemporaneously.
In many future stories, the changing of history is the villain's motivation. City of Death, for example, has the Doctor telling Scaroth that he cannot change history. However, the Doctor's attempts to stop him doing just that indicates that he is speaking philosophically rather than literally.
There seem to be acceptable and non-acceptable areas of interference, and many grey areas, and these are constantly evolving. An unmentioned, but vital, code of conduct seems to exist. Thus, the Doctor attempts to prevent King Harold from gaining advanced weaponry in The Time Meddler, but doesn't attempt to have an influence on, say, the Vietnam war, despite being on Earth during this period.