Yep. The only "rule" is that we can't change the future, except when we can.
Yeah, and there are the rare times like in "Fires of Pompeii" where the Doctor himself sets off the event that's seen as a historical tragedy. He didn't know he was the one who set off the volcano, he thought it was just a fixed point in time until he got there and realized what was really going on. I feel like a lot of times, the doctor is the reason that the Earth has survived to the point it has, he just doesn't even realize it until the events occur. A lot of them are hidden or not made aware of by the wider public, so they wouldn't be in the history books, so the doctor wouldn't know about them.
There've been lots of exceptions lately though, like (off the top of my head) the "Power of Three" episode where little black cubes invade the planet. Every person on earth is aware of them for over a year and they almost wipe out half the population. That woulde definitely make it into the history books and it's something the Doctor would have been aware of since he's visited earth post 2014... so when the cubes show up, he should be familiar at least with their historical significance, even if he doesn't know how exactly he'll be involved with the events...
Like, it's implied that 9 saved a family from boarding the Titanic (Season 1, Rose), and the invasion of the cubes would be a much bigger historical event than the Titanic, so why doesn't he know about that already? Oh well, it's inconsistent but it's fun.
Father's Day was a very special and unusual situation, there were multiple versions of the doctor and his companion in the same place at the same time and a man's death was prevented by his own daughter, who would never have had to go back and prevent his death now as he'd survived, so he would have died, so she'd have to go back and save him, so he'd live, so she wouldn't have to go back and save him.... it's a little crazy. I think the reapers haven't shown up any other time because a situation like that hasn't arisen since then.