For your safety when you use cruise control you need to put your feet flat on the driver's side floor and ready for braking or maneuvering if you need to suddenly slow or emergency stop.
Make sure your foot is ready to be able to use the brakes in the event you need to suddenly stop.
You can move your legs around though and move your foot around to keep them from falling asleep but do not curl your foot up or put your feet on the dash etc.
Always have your feet placed where you can use the brakes and accelerator when needed and you'll be okay.
Using cruise control can be a great thing and help prevent driver fatigue as well as prevent your foot and leg from going numb which can happen during long car drives.
The cruise control also helps you save gasoline and your miles per gallon will improve because your throttle will be at a constant speed.