For death in X rounds, I just make them start crumbling, the magic holding them together is no longer strong enough to overcome the injuries. So a vamp will start turning to ash/dust buffy-style, skeletons will start having bones detach and they eventually die.
The major factor for me is, how long do I want to spend in the fight? Most combats take longer than I want anyway. As a GM, remember that undead don't run away or vary tactics much, so making the fight grind down to a "must take all their HP" fest can be very dull.
One change that I am really happy with, is making all holy attacks do their actual damage to undead. So a holy weapon can make undead bleed, stun and feel negatives. This was necessary as the holy table is simply no better than a normal attack table, so I can see no point in using spells like blessed weapon. I love this change, as now before combat, all characters run to the cleric and plead for a blessing.