I am developing an Android App by myself. Using Eclipse. I did not defined any repository on the web, nor am I using CVS (there is no team). I have all the code sources locally on my computer.
How do I go about tagging the current source with a version number?
Is it customary to tag once when stable and again when ready for release?
I would strongly recommend using some sort of version control. You can get a free account on github and store your code there. But using version control is independent of how you use versions in your plugin.
This link contains the Eclipse guidelines for versioning:
http://wiki.eclipse.org/Version_Numbering
To summarize:
4, bump up the minor version (second number) for new functionality that maintains backwards compatibility