I have also used 2 other scenarios for setting clocks on my systems that I don't want to be constantly hitting a public timeserver. For one system I have it run ntpdate once a night to keep it on track but I don't put that timeserver ip address in /etc/ntp.conf -- I only populate /etc/ntp/step-tickers with the ip address. The other system, which is connected only rarely and also isn't on during the night, I have it restart ntp once it establishes a connection to the internet. So I have a script that runs wvdial, followed by service firewall restart, followed by service ntpd restart.
Also I recommend setting up one machine on your network to be the network timeserver, only that one should contact public timeservers. All your other machines should get their time from "your" timeserver.
"Please don't abuse public timeservers, please read the following paragraph taken from the ntp documentation, in order to make sure you are polite to public time servers:
Please respect the access policy as stated by the responsible person. It is very important that potential clients avoid use of servers not listed as open access, unless approved first by the responsible person. This especially includes indiscriminate use of servers not listed in the list, since this can be disruptive. The responsible person should always be notified upon establishment of regular operations with servers listed as open access. Servers listed as closed access should NOT be used without prior permission, since this may disrupt ongoing activities in which these servers are involved."