There's no one-size-fits-all answer for the subversion convention question IMHO.
It really depends on the dynamics of the project and company using it. In a very fast-paced environment, when a release might happen as often as every few days, if you try to religiously tag and branch, you'll end up with an unmanageable repository. In such an environment, the branch-when-needed approach would create a much more maintainable environment.
Also - in my experience it is extremely easy, from a pure administrative standpoint, to switch between svn methodologies when you choose to.
The two approaches I've known to work best are the branch-when-needed, and the branch-each-task. These are, of course, sort of the exact opposite of one another. Like I said - it's all about the project dynamics.
