Deciding when to upgrade Ultimento is a bit like deciding when to upgrade Magento itself. We're always rolling out new features and fixing the occasional bug and for most people we recommend you always stay up to date with the latest version of Ultimento.
That said, any time you upgrade any part of the software on your Magento installation you are changing system files and that brings with it a risk. As much as we test all our updates, every installation is unique and there's no way to be absolutely sure a particular setting or extension won't cause a problem during an update.
We HIGHLY recommend you test any update or upgrade in a development environment before you install it on your live site. Yes, it takes time and can be annoying, but not nearly as annoying as having your site break. Just figure out if the new features or fixes of the latest version are worth the hassle of testing and you'll be ready to go.