Should a mechanical power switch float or ground main power when the system is off?
Shorting the input to ground can damage some regulators because the output capacitors and multifarious bypass capacitors which may be connected to the output discharge through the regulator. It's not necessarily easy to tell from the datasheet whether that is true. You could add a resistor between the switch and ground to reduce the chances of that happening.
Some regulators have a pull-down transistor on the output to quickly discharge the output capacitors so you can reset (say) a micro by quickly cycling the power. This particular one does not seem to. Discharging the input capacitor with a switch takes you part of the way there, but there is no guarantee the output capacitors will be discharged quickly (though they probably will).