This is just my personal opinion, I am not aware of any official guidelines.
The effects of a valid flags are that the post is deleted and the user suspended for 30 minutes from chat. So you should consider the flag valid if you think these actions should happen, and invalid if you don't think the suspension and/or deletion should happen.
Obvious spam is easy, those flags should be approved so that the user will be stopped from continuing.
Offensive posts are a bit more difficult, I would consider flags for direct insults at other users to be valid. The 30 minute suspension would give the offending user some time to cool off and it might be enough to deal with the situation.
I don't think the offensive flag should be used for removing the occasional expletive, strictly speaking those are offensive, but the suspension side-effect of the flag is not wanted in these cases. Telling the user that he should tone down the language and asking him to self-delete should be enough in many cases.
More complicated cases should not be handled via these flags, but via moderator flags. The spam/offensive flags work on a per-post basis, if there is a problematic user moderators have more tools available to deal with those.