Is it a good idea to sand off the rotor when exchanging disc brake pads?
Unlike car disk brakes, there is not a lot of spare "meat" on bicycle rotors. "Machining" the rotors is not really feasible. If they are worn to the point grooves are causing performance issues, the rotors should be replaced. 

Sanding them down evenly (without expensive machining tools) will be next to impossible to remove scoring and maintain a uniform rotor thickness. It is likely to cause pulsating or even the brakes to grab or lock up.

A light sanding probably will not hurt, but is unlikely to make any significant difference.