My experience has been that occasionally you run into small bugs in whatever new goodies the team have added, but never has a release candidate caused me any undue hassle. That said, they are testing versions, so backups are always a good idea.
I generally used to set up new versions as a separate entry in whichever boot loader I was using at the time (so I'd have core.gz and core-3.1-rc1.gz for example), so I could easily switch between versions if needed, but I never needed to. The core team put out a very solid product.