I have never chocked my rig until I bought the current one. While getting it ready for the trip, I put the jacks down while on a slight incline. I was pretty sure I have the emergency brake on and I know it was in park - however the angle of the incline favored the swinging jacks moving back into their stowed position and sure enough, as I climbed into the coach it rolled forward off the jacks.
I can only think that the jacks hadn't fully locked down, but I did go and purchase a pair of chocks from Amazon the next day and will use them when I am parked on an incline, just for peace of mind.
For $10 it just seems like extra insurance to use them if you are uncertain of the rigs security - I don't think I can see a negative anywhere.