If a trained fighter is against a skeleton, most probably he will never go “for the liver”; he will rather aim for vulnerable parts that keep the skeleton standing and attacking (“Undead Lore” is a popular secondary skill among some of my players because of those situations).
The critical tables are generic, though. It's not as if "head was hit" would make any sense against a slime, for instance, a creature for which being hit anywhere would be about the same anyway. The end point is that such creatures are supposed to be "special" in that normal weapons aren't effective against them and weapons especially designed to fight them are needed (holy weapons for undead, for instance), regardless of how skilled someone is. If most of your PCs have a high bonus in
Undead lore, they should just know that and go out with holy weapons. :p
If you want to add into the equation knowledge of the creature, though, merely allow the PC to use its skill ranks (
not the skill rank bonus; or (skill ranks / 2) or (skill ranks / 3) depending on how effective you want the knowledge to be) to modify the critical roll (though 66 should remain a natural roll).
For instance, if a PC has 15 ranks in
Undead lore, he can add or substract up to 15 to his critical roll (or 8 or 5).
You may want to ask for a skill roll for the critical roll bonus to be effective but I'd go without in order not to add yet another roll. Leave skill rolls for informational needs.