I just automatically went with a warrior head, especially when finding higher level Rush Attack / Fatality heads than Fireball or Magic Missile / Magic Blast (not that it used Rush Attack much). I ran into trouble trying to kill a couple level 21 imps at level 17 (needed 4k experience to level before heading to sentinel island, so I could get a full level's worth from mindreading in the first encounter). I switched to a ranger head, and the creature was much more helpful than as a warrior. When I found a mage head, I switched again (which seemed a little better).