You can wait until you have much more content before applying a theme.
I prefer to get it done near the start, partly in case early visitors get disillusioned by a bland interface, partly because I really dislike the navigation options on the default theme, but mainly because I like to get a feel for the site I am working on and identify with it from the beginning.
I have already created 4 posts, but all these are labeled ‘Uncategorized’. This is a complete waste of time. At the very least we want a catchall category that reflects the key site concept.
In this case, the key site concept is Business Web Hosting. You should change the default category to your key site concept. In WordPress, this is in Manage Categories, and my installation wizard will automatically pick this up and change it.
