You will find several reports about people that didn't get an entry stamp at the Schengen Border. Especially CDG is mentioned quite often. Best thing to do if you didn't get an entry stamp to keep your boarding pass if arriving by plane or your train ticket if arriving by train. This way you can show at least something.

Rule of thumb about overstaying is that 1 or 2 days are normally tolerated but only if you have a clear record and the officer at the border is not in a bad mood. Still I wouldn't include overstaying in my trip since unexpected things can always happen that you need to stay longer than you actually planed.