If I understand correctly version of subversion defines:
Working copy formats are major-version specific for the client (ie, don't try to use a 1.4 client on a WC created by a 1.6 client).
Theoretically, old client/new server and vice-versa are OK, but the most testing gets done on same version (1.6.x - 1.6.x) scenarios. Generally you're OK to mix client/server versions, just new features won't be available with the older part (ie, mergeinfo is not available in 1.4 client talking to a 1.5 server, and commits from a 1.4 client would be missing that info). Again, best to stick with same major version, but you can get away with differences if necessary.
Usually you can find the svn library version that a particular client was built against in the release notes.