Yes, an update (especially if it's not a major version update) is far less likely to cause trouble. Again, no guarantees. Also, you can see the karma status of the testing package. If you're not one of the brave hacker kind, you might want to wait until the package accumulates a little karma and check on the comments of other testers. If it has not bombed on 5 other installs, it's less likely (but again, not impossible) that it will bomb on your install.

Your biggest danger is actually pulling in something as a dependency (say, that updated Xournal depends on a newer version of a library), and that dependency causing trouble with already installed packages.

So, testing is not that seven-headed monster that requires full-body-hacker-plating, but make a backup before you enable it just for peace of mind, turn it off after you installed the app you want to check, and don't curse developers (too loud) if (however (un)likely) something goes wrong - we're human, too