You can create one whenever you want, if you feel the need, but you certainly do not need to.
It doesn't matter at all, as all the hostnames you create on a given account and MySQL server are "interchangeable" (to quote the web panel terms).
They are all aliases for one another.
Some like to have hostnames for every domain/subdomain, but when you add a new one you have to wait until it is available via DNS, and you can use the existing hostname without that wait.
Really, it's all up to you!
--DreamHost Tech Support