The title says it all: Are questions about best practices on-topic?
To give some context, I think that it is fair to ask the question in tex.stackexchange.com why the commonly cited best practice in TeX editing to split your document into sub-files for the chapters/sections is actually a best practice - I can think of 10s of good reasons for not doing so, but only of two for actually doing so. The question would list the reasons for either way as a starting point.
I believe that they might be on-topic as answers will consider valuable information for practitioners. They might be off-topic as the question is quite open-ended and it is probably not suitable to pick a "right" answer.
My opinion, which need not reflect other's, is that such questions are welcome, so long as they are about TeX and friends. As the distinction is somewhat blurred, I'll try to make some examples.
“Should the Introduction or Conclusions be numbered chapters?” is not about TeX and friends
“My institution requires double spacing, how should I do?” is about TeX and friends, because it's possible to use packages (setspace) or classes (memoir) for dealing with the problem in a non hackish way
“What is the best sans serif font to accompany Bookman?“ is not about TeX and friends. The answer is, of course, “don't use Bookman in the first place”. The only time I used it was for printing my brother's homework which needed to be two pages long and with Times it was only one and a half.
“What's the best way to organize my document if I want it split into subfiles?” is TeX related, because it has to do with how TeX programs look for files in subdirectories and in the whole tree. While a bit opinion based, different answers can give ideas for the best setup in a particular case.
“How should I define personal commands and environments?” is definitely a welcome best-practice question, provided it tries to narrow the boundaries: not too generic (that is, unanswerable), but showing definite situations such as (but not limited to) operator names, repetitive structures, language changes.
The green tick we like so much is not for the “right” answer, but for the most helpful one for the questioner. What is important is the usability of the answers; if a question doesn't admit usable answers, then it's off topic.