I just read the discussion on Area 51 on flagging. It seems like a pretty agressive thing to set a flag, and I have trouble imagining that someone would post something offensive on Mathematica SE. On the other hand you get reputation points for flags. Is this something better left to longer time participants?
Flags are appropriate and appreciated when you find things like answers that are not answers at all but continuations of a question, or duplicate questions that should be closed or merged. It is also reasonable, though in my personal opinion less useful, to flag comments that are obsolete or of no apparent value to the community (I define friendly interaction, even if not strictly informative, as valuable to the community).
You should be very selective before flagging a question or answer as Spam or Offensive; it is not appropriate to use these flags for posts you just don't like for one reason or another.
It is also not appropriate to flag posts merely to communicate with the moderators as this leaves a permanent record of the flag on the author's profile.