A clean install is definitely the way to go ... although I've used Ubuntu's in-place upgrades between minor versions (e.g. 11.04 to 11.10) it just makes me nervous about what will break... and when.
A combination of Mint's backup/restore tool (which will backup both your files and your installed packages), and specifically *selectively* bringing subfolders from .cinnamon and .config back into your profile after reinstalling is a fairly painless way to go.
To the point made here, an upgrade isn't necessary by definition, unless you're running an unsupported OS or want the features of the new version (e.g. hardware support in the kernel, desktop manager tweaks, etc.).