My folks drive to France several times a year - they used to put the stickers on, but haven't always done it. They haven't had any trouble though in all the years they've been going - I have heard that some French police can be a bit over-keen, so i guess it's pot luck. The way most French people drive, you'd be very unlucky to stand out!!
What you could do at the very minimum is have the stickers in the car with you, and then if you did get pulled just play dumb and say you forgot to put them on.