Ok, so when I am looking for diamonds I strip mine at level 11. There are a couple reasons why i mine at this level. One is that it is the level that most diamonds spawn on. Also 11 is the layer that most lava pools are at the top so when you do a mine if you want it to be neat then you will not have to mine through the obsidian. I just think thats looks of people have there own ideas so I just wanted to share what I do!! :D