It's difficult to overstress how important good time synchronization is to network operations. In chapter ten of the book Hardening Cisco Routers, Thomas Akin has this to say:
- Time is inherently important to the function of routers and networks. It provides the only frame of reference between all devices on the network. This makes synchronized time extremely important. Without synchronized time, accurately correlating information between devices becomes difficult, if not impossible. When it comes to security, if you cannot successfully compare logs between each of your routers and all your network servers, you will find it very hard to develop a reliable picture of an incident.
While Thomas has a complete chapter on how to set up NTP on a cisco router and how to configure that securely, and he tells you why you want all your routers to be NTP clients, he doesn't tell you about some of the problems of using them as NTP servers. For that, you need to go to the page Distributing time from your routers at the NTP Public Services site.