If I understand correctly version of subversion defines:
You are very close to the mark. Working copies and repositories are always readable by new clients using the same major version, so a 1.6 client will always be able to handle an older server or working copy.
However, you do still need to be aware of the version supported by each one. Most will automatically upgrade a working copy automatically to their own version; so if you checkout using an older 1.4 client, and update it with a newer client ( eg. the latest svn 1.6); the working copy will no longer be usable will the former.