I read somewhere, that cross posting is not allowed or encouraged between SE sites.
Official faq question:
Is it possible to post questions across a couple of the sites because it's debatable which site it fits best?
1) If I have questions on Google Earth, I'll want to post it both on the SuperUser and GIS.SE sites. The faq of both sites considers this a valid question. SuperUser community accepts any valid software question and GIS.SE will take any valid GIS software question.
Geographic Information Systems - FAQ says:
The Geographic Information Systems Stack Exchange is for is for questions concerning geographic information systems and science. We welcome cartographers, database administrators, geographers, programmers, and anyone interested in or using GIS professionally.
Super User - FAQ says:
Super User is for computer enthusiasts and power users. If you have a question about computer hardware or computer software then you’re in the right place to ask your question!
There are two things to note here:
We are focused on expertise within a topic, because it is related to consumer software doesn't mean it's on-topic.
We are also focusing on solving problems, a GIS problem is different from a computer software problem.
Let's examine this example question:
It clearly requires expertise from GIS, it would be closed as not constructive on Super User.
It is clearly a GIS problem, as accuracy is required. There is no problem with the software itself.
Feel free to come up with another example question, but there should always be one of both communities jumping out. Still, if you can't decide. You can try one community and let it be migrated to the other community through a mod flag later...
2) If I have a question on ArcObjects (an SDK for the ArcGIS suite), I'll want to post it on Stack Overflow and the GIS.SE.
I believe you will find much more expertise and activity on GIS.SE, on Stack Overflow your very specific question will suffer under the flood of activity which scrolls your question of the list. Unless they are displayed on the interesting tabs of GIS experts, which would most likely be on GIS.SE too...
3) If I have a question on Ubuntu options, I'll want to post it on Ubuntu, U&L.SE and SU.
Please note that Unix & Linux - FAQ states:
Note that Ubuntu posts are a special case. If your question applies to Ubuntu only, or you're looking for answers that are Ubuntu-specific, you should post it on the Ask Ubuntu Stack Exchange site.
So, this leaves you with Ask Ubuntu and Super User. Here it is a matter of the spirit that you want to follow, look for yourself which community is best. Please note that people focused on answering Ubuntu questions are more likely to be found on Ask Ubuntu.
4) OSX questions on the apple.stachexchange and SU and U&L.SE.
OSX is not necessarily U&L.SE, so unless you have a low level U&L.SE question you shouldn't be posting there. As for Apple against SU, it's again a matter of the spirit that you want to follow, look for yourself which community is best. Please note that people focused on answering OSX questions are more likely to be found on the Apple website.