Yeah. Underscores should be avoided at all costs. In theory they're invalid in the RFC's, and a some DNS servers won't allow them. Where they are supported, they tend to be used as special characters for finding services. e.g. _ldap.your.domain.
I don't believe the two references quoted above are necessarily at odds with each other. Much as appreciate a good like MS bashing where they go off and ignore established standards, whoever wrote the article there is being pretty good. Saying to avoid underscores for hostnames, but that they do tend (i.e. Have been proposed) to get used for services to avoid clashing with host namespaces.
H