Do client and server versions need to be the same?
Can a 9.x client run against an 8.x server, or do both client and server versions need to be the same?
Yes, 9.x client can run connecting to RM 8.x ( I ran 9.1 and 9.2 client connecting to for RM8.3 during upgrade). Please bear in mind that several new functions / features only available 9.x will not work or work with odd outcome, hence this mix mode is only recommended for short period of time with core /basic functionality to get the customer going while they upgrade the server. You may also find some odd behaviour in Office integration including Outlook as well as Check In Styles subject to your implemented model.
Clients are backwards compatible, so yes that situation is possible. But as Harry noted, this is only meant to be a short term situation around upgrading. Ultimately, for full support from MF Support, you'll want to have client & server on the same version/build. There are certain unavoidable siuations where a single Workgroup Server made need to be patched or have a hotfix applied, but it's usually going to be a WGS that doesn't require client connection (web server, sharepoint server, event processing server, etc).
In general though, they should be the same versions as you can read in detail in the CM9.2_Install.pdf.
CM Support Team Lead AMS