If you're using Ubuntu but have a question which is applicable (or you think it is) across distros is it considered best practice to post in Unix SE?
Or is it more a case of the question being valid in both communities with an overlap in content?
Most questions that are on-topic here, are on-topic there too.
But it doesn't go the other way around. For example, questions about Linux Mint are on-topic there, but not here.
And if you post Ubuntu-specific questions there, you might get a comment saying something like "You might find more expertise on Ask Ubuntu".
I recommend that you ask Ubuntu questions here. (Even if you think the same question may also apply to Mint, Elementary, etc.) If your question is about Ubuntu (or Xubuntu, Kubuntu, etc.), ask it here.
Note that http://askubuntu.com is specifically dedicated to Ubuntu questions.
Questions for Kubuntu are probably better asked at https://askubuntu.com/, the stackexchange site for Ubuntu.
Speaking as a Unix & Linux moderator, any question about Ubuntu Linux would also be on topic on U&L. The scope of Ask Ubuntu is almost a subset of the scope of U&L. The exceptions are things like the Ubuntu phone and Canonical services like Launchpad. That said, the folks on U&L tend to have little interest in GUI stuff and the vast majority don't use Ubuntu and certainly don't use Unity (a desktop environment which is not very popular outside the Ubuntu world).
So, while you can post your Ubuntu questions on either site, if the question is about Unity or, more generally, if you really want a GUI answer, you are more likely to get an answer that meets your needs here.
To give an example, if you ask about configuring a network interface, on U&L you will probably be told to edit text files while on AU you will most likely be given screenshots of the various GUI dialogues you would need to go through. Both answers would be correct, both would actually do exactly the same thing, but which one you prefer is up to you.
So, in conclusion, the only "best practice" is i) never post non-Ubuntu questions here and ii) post Ubuntu related questions on whichever of the two sites you prefer.
I'd like to expand a bit on what @terdon wrote above. As a frequent poster on U&L, we see all types of questions from Scripting Questions to configuration questions to network questions, and then the occasional theory questions. I linked that question to show that those answers come with experience and use of Linux in general.
Posters there tend to be experienced regardless of distribution, and therefore we tend to lead answers into the non-GUI realm, because with experience you find that it's easier to edit the config files, than it is to "fight the GUI."
Honestly, we don't care what distribution you use, but realize that when you post a question there that what some of us may tell you to do to fix your issue may be outside of your Ubuntu comfort zone. If you, as a question asker don't feel comfortable expanding your linux knowledge by stepping outside your comfort zone at this point, then the question belongs in AskUbuntu, because Ubuntu has GUI support for nearly every major tool or task.
On the other hand, if you're ok editing files with elevated
sudo commands and
vim, and reading comments, along with man pages for commands you may have never seen, then we invite you to read and post questions.
If you are running an Ubuntu server, my experience is that AskUbuntu is the better source of definitive answers, because they necessarily account for conventions inherited from Debian, and conventions specific to Ubuntu.
Answers at other sources may (and sometimes do) reference conventions specific to other distros without realising it. If you want to understand the broader GNU/Linux landscape, that can be interesting or helpful, but if you want to execute a task on your Ubuntu server right now, AU is the more direct route.