Originally Posted by B747-437B
The official rules are that every non-EU citizen MUST have an entry AND exit stamp from the Schengen area - but you can't find an immigration officer on half the land borders with Switzerland with a magnifying glass if you tried. The onus is also on the traveler to prove that they have not violated the 90/180 rule, which must be done via passport stamps. I have never had an issue (I average around 50 Schengen stamps per year), but I do know someone who was quizzed by the French at CDG (she spends ~150 days/year in Paris and Italy).
I've entered Shengen at CDG on a US passport nearly 100 times and I've been stamped maybe twice. I've exited Shengen well over 100 times at a variety of locations and no one has ever scanned my passport for when I entered. YMMV.