Reality: 90% of on-street parking spaces in Paris are free of charge this month. Meter maids are still on duty, though.
Here is a list of streets, by arrondissement, detailing whether parking is free (gratuit) or still requiring a payment via Moneo or Paris-Carte (payant).
Tip: I don't own a car. But the best advice I've heard is to check at the nearest borne where you would normally buy a parking slip for your dashboard. Make sure that you've landed in an otherwise legal space (not for handicapped, deliveries, etc., which are usually well indicated.) If when you push the yellow horidateur button for payment it says "le stationnement est gratuit," -- bingo -- you're in luck!