My standard route has been to go to confluence, find the docs sections, then navigate through to the install docs for the version, e.g. sakai 10:
https://confluence.sakaiproject.org/x/iYGLBQ
Through one means or another I happened across the source route to this too, so starting here....
http://source.sakaiproject.org/release/
You get redirected to the latest stuff, and appended version numbers to that url gives you other docs, e.g. adding 2.8.2 or 10 to the end of the url
But the links to what I should download are quite often not there, at the time of writing the 10 tar ball and zip in the confluence links are dead and the source.sakaiproject links doesn't have the 10 docs yet (redirects to 2.9.3) presumably this is because v10 is not released yet....
So, I'd like to evaluate a new version of a sakai source install, what's the best way to do this? (considering the official documentation for install is still being formed)
Do I download the latest SVN, or the latest RC or the latest beta or??? How do I know what's best to test against without being "too" bleeding edge? Is there a recommended tar ball/zip link to test against? Is there a "latest good" SVN branch?
The latest code is always in the Sakai trunk (currently svn):
https://source.sakaiproject.org/svn/sakai/trunk/
That code may very well not be stable though as it is where things are being actively developed. If you are not actively developing then you should stick to the releases as indicated on the project website here:
http://sakaiproject.org/current-release
If you want to use something in between (say an upcoming release) then you can grab the most recent tag or maybe use a recent branch (both currently in svn, latest shown below at the time I write this):
https://source.sakaiproject.org/svn/sakai/branches/sakai-10.x/ https://source.sakaiproject.org/svn/sakai/tags/sakai-10-rc02/
The reality of the situation is that if you want to use something other than the release then you should really participate in the dev community for Sakai. Joining the mailing lists and the weekly calls will provide the information you are asking about and much more.