I don't think there is any hard-and-fast rule to say when you should create a user and group for a particular program. If I were forced, I'd say each daemon, especially one with the ability to acquire root-like privileges (switching users) or has a large amount of data to read/write, should be run under a unique user/group combination. The opposite end of the spectrum has a user/group for every piece of software installed (for a unique method of package/software management) - see Linux from scratch
; I think "pkguser" is the name of the package management method that does this if you're at all interested.
As for pixellany's response... well, I think there are times when people don't really read a post to understand exactly what's being asked or jump to a likely conclusion. It happens... it's happened to a number of my posts as well. Don't sweat it - unless the response causes the thread to get hijacked.