I admit to having gotten into a little bit of a comment argument with someone last week because stated that something was a Rule and they countered, insisting that it can and did get broken all the time (if memory serves, the Rule I invoked was "write in past tense"). The thing is, I don't think we actually disagreed on the subject, except for my use of the word Rule.
The thing is, any semi-experienced writer comes to realize that there are, in fact, Rules to writing. Now, none of these rules are inviolable (except maybe the one that says "make sense", and I can even think of a ton of examples where this one was successfully transgressed), but they do exist, and breaking them has consequences. Personally, I find that having a set of heuristics that I can choose to "break" from time to time is much more useful than being supposedly "free form".
So... why am I choosing "rule" instead of "guideline" or "heuristic"? Because I think that by and large, things like "use past tense" or "don't use second person" are rules in the sense that there are consequences for breaking them. That doesn't mean they can't be broken, just that they exist and there are consequences to breaking them. The best parallel I can think of is in music theory. There is a "rule" that says that when you play a V chord you have to follow it up with a I chord. If you don't, then classically speaking you don't close the loop that the audience expects you to close. Now, even back in classical times there was a way of circumventing this called the deceptive cadence, and with rock in particular this rule is violated (example: "Louie Louie"'s chord progression is I-IV-V-IV) so often that it's hard to even notice anymore, but if someone was writing a classical choral work and wanted to know if it was OK to go from a V chord to, say, a iii chord, I'd have to say "well, the Rule is that V has to go to I... and the consequences for breaking this rule are X, Y, and Z".
I hope that made sense...
I figured I would open this up to meta-discussion. Personally, I think that "hey, you can do anything you want so long as it works" isn't terribly useful for beginning writers, who I assume are likely to be the people who come to a place like this. But I'm willing to be convinced by a solid argument the other way.
Do these kinds of answers to questions have a place on writers.SE? Yes, we can always just say "meh, let the people decide," but one of the points of a SE that's still in beta is to help determine these things.