After entering Schengen at CDG countless times (over 100 for sure), I never ever was stamped. Yet recently it happened for the first time. But I think the officer was distracted. She was talking to someone else while reviewing and stamping my passport. However at other French entry points such as LYS and NCE they have always stamped, going back years.

At CDG, it seems that off and on certain officers look at the passports or don't look at them. Often they just wave you through as long as you have a US or European passport. Other countries generally get scrutinized as most of them need a visa for France.