Re: 7X05 The Angels Take Manhattan (Grading/Discussion) (SPOILERS!)
Granted, Doctor Who time travel rules are never consistent, but the one rule that always occurs is that it's bad to alter your own timeline. Here, they explicitly said that what Amy and Rory did could easily have destroyed New York city and that doing it a second time definitely would have. They "got lucky" that things worked out the first time. That's why you don't want to risk a second time (especially a bigger paradox).
When on Romulus, Do as the Romulans