It's not really a function of the RTC client version but rather the underlying Eclipse version that will cause you problems.  Eclipse writes metadata to the workspace directory and checks against the client that tries to open it. If the workspace is from an older version of Eclipse the metadata will be upgraded. In the opposite case, Eclipse will issue warning messages.

Having said that; it's not normally a good idea to share workspaces with different versions of Eclipse.  See https://jazz.net/forum/questions/82339/is-it-possible-to-share-an-eclipse-workspace-between-two-different-versions-of-eclipse-at-the-same-time for a reason why.

As to the second part of your question; just knowing the RTC Client version doesn't necessarily provide you with the Eclipe version.  It is possible to have Eclipse 3.6.2, 3.7.2, 4.2.2 ... all hosting the same version of the RTC Client depending on how it has been installed (Installation Manager, plain .zip file or p2 repository plug-in).

From experience; I always give each new Eclipse installation I create it's own unique workspace specifically to avoid these types of conflicts and just import the settings and projects I need to work on to that instance. This approach means that any SCM assets I load from the RTC repository are using an RTC client of the same version as the server which provides an extra level of stability.