If that is the way your pay is structured. When it is, I see most employees charging 40 hrs regardless of whether they worked 36 or 44 hours. It is disheartening. Not to mention it completely screws up the metrics.
When a manager tells me to charge 40 when I work 48, I refuse. If they want to hit their budget/numbers, then I usually work in a paid day off, because they can't stomach reporting the actual numbers. I had one that tried to force me. I took it to the ethics department. It was squashed immediately.