I couldn't really say.

Surely if you were to slap some content at "whatever.domain.com", plastered with links to "domain.com" (your main domain), the search engine would be smart enough to realise they are part of the main domain, and not consider those links when tweaking your search engine ranking (or at least not value them as much as links from completely different domains)?

On the other hard, they may be recognised as keywords, so I imagine they can't do any harm.