The source code is tagged with the version number just prior to release. However sometimes the releases are patched with bug fixes after the initial release and those fixes would not be in the code as of the tagged revision. The download page on codeplex shows the changeset number of the release that was tagged with a version and has notes about any patches that happened after the release. There should also be comments in any commits subsequent to the release that have bug fixes.
The default branch is generally always intended to be stable, only rarely will there ever be a problem updating to the latest revision of the default branch. Right now for example there could be a problem if you are also using some of the add on products because the source code has been updated to use the newer version of ajaxcontroltoolkit but the add on product releases will not be updated until the next release of mojoPortal. So deploying a build of the latest source code at the moment could break add ons that use the older version of ajaxcontroltoolkit. But again this is a fairly rare occurrence in the development cycle that only happens when we upgrade to new versions of libraries that both mojoPortal and the add on products depend on.
Hope that helps,