The testing repo is for packages that haven't been tested on enough computers to determine they're safe to offer in the regular repos. I have the testing repo listed as a repo to always include, but I *always* notice if a package comes from testing and make sure I have a sufficient reason for installing it.
You need to be aware that testing packages are being *tested*--that's why they're in testing. There could be risks, though a package would be very unlikely to trash your whole system. If you're willing to accept some risk, by all means enable testing. If you do install a package from testing, please report your results, good or bad, to the VL package news and Updates section of this forum under the announcement of that package. This helps get packages out of testing into the main repos.
I don't think I've ever had bad results from a testing package, but I don't test anything but those for which I want the newest version.
One thing you should NOT do is include the testing repo if you're doing an "automatic update," where you mark all upgrades and then execute. People have gotten into trouble that way.
I hope this answers your question. Don't be afraid of the testing repo but exercise due caution.