We've all heard of house rules that fix what isn't broken; a DM has misunderstood how the rules work and so applied a patch that causes more problems than it solves. Or has allowed something that he knows is technically against the rules, but doesn't seem to be problematic. You can imagine how well *that* turns out.
Sometimes that DM is me.
Mind you, I am not as bad as one reading my house rules posts might think; I examine lots of rules variants, but rarely depart from the RAW in my own games. And those are considered house rules; house rules made up on the spot is something I try to avoid if at all possible. After all, there has usually been more thought and playtesting and experience invested in someone publishing a set of rules than there is in me reading them and then immediately trying to apply them to a game session. What I try and do is read rules in advance of a situation, and think of ways that they might have undesirable consequences. If they do seem to be "broken" then I have to verify if it is the rules at fault, or my understanding of them.
So if I am not sure I understand a particular section of the rules and their interaction with other rules, or if I have not yet formed a judgment on them (e.g. is
miasma broken, or just powerful?), I'll ask about them in the rules forum. It's a way of being alert to potential problems that might arise in game, and being prepared to house rule if necessary. But if carefully following the rules as written avoids a problem, it is aesthetically displeasing to make an unnecessary houserule.
I, like Henry, use the term RAW to refer to any sort of published material with its erratta. Not just the 3 core books. (Sorry Infiniti2000!) Like Patryn of Elvenshae I generally don't include RotG articles, since they often contradict what is written in the rule books. Likewise for the Sage Advice FAQs, and, of course, WotC customer service. I would say that Hypersmurf's approach to rules questions best exemplifies what I am asking for when I ask a question about the RAW. Though it sometimes borders on the pedantic, he has the kind of sensitivity to wording and context that I admire.
