Do I have to remove the CF memory card from my digital camera when it is not in use?

I own the Canon Digital Rebel XTi that takes a CF memory card. Will the camera or the CF memory card be damaged if I leave the memory card in the camera while the unit is not in use?

Or is it a good idea to remove the memory card upon shut-down?

Nope. You can just leave it in there.
In fact - the fewer times you remove and replace it, the less likely the pins/connectors will wear out.

leave it.

Nope. The best place for your card is in the camera.

I would take it out as little as possible, less chance of losing it.

nope, leaving your CF in camera has nothing to do with damaging it.

no....it's safer inside the camera

no it is safer in the camera, with less chance of losing it or doing damage to it.