Easy, become a sheep and use Twitter Bootstrap like everyone else because it doesn't matter what the reasoning for a labels location is. Twitter will know best.
If you have your heart set on making your own css from scratch. Have a look through this article, it'll help to explain the pros and cons that will benefit users using your form. Just take into account that page width can change and adjust your form accordingly.
How do you put all that disparate information together to make a good decision, especially if you’re in a hurry?