As far as I know the its ok for the car to move when its in park but no to much.
When the car is parked on a flat surface then it wont move after putting the auto-transmission on park and release the brakes. But say if you park on a incline position then when you move the knob of the gearshift on P the car will move like 3-4 inches(not exactly sure about this,might be less or more).
The other thing is that you wont be able to pull the key unles the trans is on P. They're connected, And for what I have no idea but t might be for safety reasons.
And another thing once my key got real stuck on my W140 I mean there was no way for me to get it out. Then after moving the steering wheel a little which was real hard at that position I finally got it out. The problem was that after I parked on this bad spot (only onw I could find then) the steering wheel was left in a real bad position and the only way to get the key out was to use a lot of force to move the steering wheel left or right for that one inch(or maybe less) as much it will move when the car is in park.
Good luck.