It should be noted that the auto props method has to be configured on each SVN client being used. So when you're setting up a new developer, or an existing developer on a new machine, you have to remember to perform this configuration.
If you are all using TortoiseSVN, you can set the tsvn:autoprops property on the base folder of each checkout and it will be honoured by all TortoiseSVN clients.
If you really want to nail it, you'll need to put a pre-commit hook in each repository. The enforcer script might be easily tooled for this.
If you have any python-fu, RepoGuard (the successor to SVNChecker) looks like it could be useful too.
No matter which you pick, there's no way to retroactively apply the property to existing files in the repository, I think. You can probably enforce it on the next commit of the file, however.