The MySQL "user" table that controls who can access your MySQL process contains a "Host" field that controls what hosts can connect as that user. No matter how you create MySQL users, when using our service, you should always make sure the "Host" field contains only the % character.
Since we use a clustered network with specialized nodes for hosting and specialized nodes for MySQL, you will never connect to your MySQL process from the server it's actually running on. Since things move around, you won't even know in advance which server will originate the connection. Using the % character allows connections from any of the servers on our network, which is the only way you will be able to connect. Using "localhost" or any other value will lead to connection problems.