Check out a side view of the F-4 Phantom (classic example of a/c with tailplane anhedral). At low alpha the tailplane sits up above disturbed air from the wing. If there was no anhedral, all of the tailplane would be severely affected by dirty air from the wing at high alpha (required at low IASs because of this wingplan). It's a compromise to get the best of both worlds.
[This message has been edited by Skycop (edited 21 April 1999).]