As I mentioned, with Mercurial or any version control, you can specify the precise version that you want all of the files in the "working directory" to correspond to. In fact, after doing "hg pull", your files will be unchanged. It's not until you "hg update" that your files actually get updated. And you can specify what changeset or version tag you want to "hg update" to. For example "hg update R-1.94.1".
Also, many people do want to try out trunk to verify fixes. With version control, it's very easy to reliably switch back and forth.
