The Libgdx nightly builds have a lot of API changes accumulated in them (its been a while since the 0.9.8 release). Mario & Co. try to document any such API changes in blog posts, so if you're following the nightly builds, you'll definitely want to keep up with the blog. The "CHANGES" file in the nightly distribution also has notes on what has changed (but I think this may not have some early changes in it).
Its hard to know what version a 3rd party blog or article is talking about if they don't explicitly list the version. You can sometimes go by the date on the article, though.
Anyway, I think its safe to use either the "stable" build or the nightlies. There are non-trivial API changes when switching between the two versions, so you'll want to stick with one for a while, but these changes are almost always a good idea for all involved. (The Libgdx maintainers don't take API changes lightly, but they don't rule them out either, yet -- presumably that will change with version 1.0.0.).