Steve Jobs famously said, that our society and its rules were made by people not smarter than you, which means you can break the rules, and, thusly, change society for the better.
I think this applies to smaller contexts as well. In marathon training there's the belief (without solid proof) that the long run should never exceed the marathon distance, since it would take too long to recover. I've seen instances to the contrary, where people ran a marathon or ultramarathon in preparation of a marathon, only a few weeks before.
I suppose everyone is different, and different rules apply to different people, or even to the same people in other stages of their running career. There's clearly a lot of armchair coaching going on, based on “anecdotal evidence,” which isn't proof of anything, as we should know by now. It's highly susceptible to bias.