It depends.
Do you need features in Yosemite or apps that require Yosemite? In this case we're talking about Office. If you want the new version of MS Office then your hands are tied. You need to upgrade pure and simple.
I'm already on Yosemite, its been fairly stable, I'm happy with it. I've not been bitten by some of the bugs others have. Yet, I understand the fear and loathing.
It all boils down to weighing what is more important. The new version of Office and a less stable OSX or running an older version of Office but you're on a version of OS X that is more stable and/or looks better.
I've said this elsewhere, those who resisted upgrading face this issue, some sooner then others. If you choose to stay on an old version of OS X, you are painting yourself into a corner. Eventually there will be apps that you want/need that will require the newer version. I think this is probably the major app that is causing the most consternation at this point.
Good news or bad news, we'll be hearing about 10.11 in 3 months (and hopefully it will be more of a bug fix/platform stabilization version) and then in the fall seeing it roll out. In other words, you'll be faced with a similar question come September/October. To upgrade to the latest version of OS X.