In the official example from Debian (http://www.debian.org/releases/stable/example-preseed.txt) it says:

```
# NTP server to use. The default is almost always fine here.
#d-i clock-setup/ntp-server string ntp.example.com
```

So, IMHO there is no need to mention it and I'm also pretty sure that this line was configuring ntp-server to ```ntp``` so in the best case scenario it would end up on ```ntp.YOUR.DOMAIN``` depending on ```search``` parameter in your DHCP configuration.

What do you think ?