Pop quiz: How low can you go??
I thought we had reached the depths already, but you guys managed to find a pit, leading down deeper!
To return to the question: I want to have a full cleric in the party at all times (this may have to do that the character I use most often in RPG's and who has given me most satisfaction is a female Cleric

). In BG 2 the healing is not as much required as it was in BG 1. Not only do you take less overall damage, but you also have more regenerating items. The role of the Cleric is mainly supportive: a Cleric heals and buffs any party members, and only rarely should be a front line fighter - at least not of the same status of a warrior or similar. And if a Cleric becomes offensive, he can cast some spells, which are especially effective against undead.
I heal party members in combat only when they drop really low on health (when their image starts to drown in blood

). Otherwise, I wait till the end of combat, and then remove any negative lasting effects (Level drain, etc...) and heal up everyone.