I've had clients who literally made a screenshot of my coded website and placed it directly over the design(PSD) to see if it was pixel perfect... I've given up on making it pixel perfect, because it just takes too long to get it to that level of perfection. And as mentioned, responsive design throws even more factors into the mix.
Luckily, I'm designing the company website (my full time job), so I have full control over both design and code ;)