I use to never think about this but recently i've taken to checking my machine whenever i move it.
I guess with a slot based drive it should be okay *shrug*
with with tray loading I have noticed with some that when moving the machine ..or tipping at certain angles the cd in the tray would become dislodged ..
I came clsoe to probably wrecking a cd or two when i ejected the tray and was almost ready to insert when i noticed a cd just floating inside.
So i guess never leave your cd in when moving
