Hi.
We use NTP with an ALLOW ALL policy. This means we don't block any traffic, but it is still inspected for maliceous content/activity.
For us, we see no difference in performance or server load. Admittedly, we have not taken any measurements with tools on a before & after basis. This is more from talking to users and our own experience.
One issue we had that NTP made worse is ghosted NICs on VMware guests. See this MS article if you want more info: http://support.microsoft.com/?kbid=269155