Well.. in my opinion assuming character's death as something "positive" in a challenging campaign is a mistake.
Characters death is the result of precise errors or shortcomings by the character itself. I think a well balanced campaign should not consider a "death count" the measure of how much challenge it provided.
If the party has to work hard for his survival and can feel threatened by the obstacles it face then is a well balanced campaign.
Of course, character death should be a serious possibility... You zig when you should have zagged, you suddenly fail in a critical task .. and you should Die.. Sure.
But the difference between a quest and a suicide mission is that the party is considered able to face the threats ahead and turn back home with their own legs.
So I don't think deaths are a viable way of measuring the balance of the game ...
On the other hand .. if a party can make a fool of itself, do all the wrong tactical choices and still complete the quests with no repercussions .. that's the signal...