I played a guy today who would start every deuce serve with his back foot across the imaginary extension of the center line. He even still so close to the center mark on ad serves that I'm sure he crossed the center line often as well on those serves. This seems to give slightly more of a service advantage than foot faulting the baseline(unless you're a serve and volleyer) as you end up with more of the court on your forehand(for a righty).