It's a grey area but it's also one of the reasons for the Stack Exchange site existing. There will definitely be questions like is X approach better than Y approach or what is the recommended way to do Z.
A lot of topics - e.g. Navigation, have several ways to approach them and each has their pros and cons but as a newbie, these pros and cons aren't necessarily obvious and it would be good to have them explained somewhere.
I guess it comes down to what we want the goal of the site to be, are we for disucssion of Tridion knowledge a la Programmers or are we for answering specific questions like Serverfault/StackOverflow.
Could we make use of the Community Wiki functionality here?
I'd say we keep an eye on it and watch out for argumentative questions e.g. "Why is X better than Y?" but allow for "What is the recommended way to do X?" as long as the answers give all arguments?
This is a hard question.