Safely merging multiple MySQL users to %
First off, i'd suggest you avoid using % wildcard (if it's not strictly necessary). If your users connect to the database from the same host the mysql server is running on, my advice is to use 127.0.0.1
All best practices point out that no-password login should be disabled.
As for IPv6, there's no point in having that user if you're not using it.
I'd suggest you read this http://www.greensql.com/content/mysql-security-best-practices-hardening-mysql-tips