I havent tried 18/17 combo, but 17/16 worked fine, and as you say, it is supposed to work, and i would not hesitate setting this up. PDM is guaranteeing backwards compatibility 3 years back. More than that you start getting problems with add-in in SOLIDWORKS.
Like Martin Solem we haven't yet upgraded PDM to 2018. However, we always have some period of time between our PDM upgrade and SOLIDWORKS upgrade and have never had problems. In fact, we ran PDM 2017 with SOLIDWORKS 2015 for a while.
We're running that combo. No issues to speak of after ~6 weeks
We are evaluating an upgrade to our system so I have bookmarked this page to stay tuned to this thread.