I have a friend who only changes diapers on her daughter when they are full. Based on what I've seen, full = major sagging diaper. She also has been battling bad diaper rashes, bur doesn't see a correlation.
For me personally, I change anytime I feel they are wet at all unless we are out and about where its hard to get to a changing table. So I go through a lot of disposables when I use them.