You are correct with your second statement... That should do the trick...
You could also do something else, that in case an error occurs you will get a message..
then your command would be:
dpkg-reconfigure ntpdate 2>&1 1>/dev/null
This causes error-messages to go to stdout, and stdout to go to /dev/null
Since stderr (2) was defined before stdout (1) was, it will actually go to stdout and not to /dev/null
If this is too confusing, just use:
dpkg-reconfigure ntpdate >/dev/null