It boils down to what properties you want your analysis to have. Cox and Hinkley's "Theoretical Statistics" has a great discussion (section 2.4). Basically, you might want your analysis to have a certain kind of internal consistency. But you might also want your analysis to be replicable either by yourself or by another researcher. Those both seem like pretty important things! But there are edge cases (like the early peeking example) where you can't have it both ways. So you have to pick which one you want, and use the corresponding methods.