I'd advise against running testing if you want stability. I get a lot of breakages on my Stretch laptop with routine upgrades. When I look into them, a number of breakages seem due to transitions to newer library platforms where all the kinks haven't been worked out yet. Other packages simply have more systemic issues which will take a lot more effort to fix.
For example, on Sunday I upgraded the xserver and wayland packages to their current testing version. Now my computer freezes or hangs when it tries to start KDE/SDDM. Can't switch into consoles and the only option is to hard power off. Seems to be a problem unique to me because I can't find anybody else complaining about it. Again, though: routine upgrade. I've also had problems with btrfs completely locking up when a partition gets full and nftables doesn't work right now because of a library platform transition. Worst of all, reportbug-ng's sending gibberish reports to the bug server so all my reports get summarily rejected! So it all depends on how cutting edge you want to be. The 'core packages' seem to be holding up. I hope that helps.
