Different distributions do different things. Pclos has some guest additions installed by default (or did once), but not all. I'm afraid I don't know what the other distributions do.
The place to ask that is on their forums if they haven't got wiki or FAQ pages to answer those questions. If you think about it, the question you're asking here is how those distros arrange (or not) to install GA in VB. That's nothing to do with pclos at all. They do what they do.