Whilst you can quite happily use the domain name servers located at your ISP, you can also set up a local caching only (secondary) name server that is brought up by the ip-up script. The advantage of running a local (caching only) name server is that it will save you time (and bandwidth) if you frequently contact the same sites during a long on-line session.
DNS configuration for a caching only nameserver (that uses a "forwarders' line in the named.boot file pointing at your ISPs DNS) is relatively simple. The O'Reilly book (DNS and Bind) explains all you want to know about this.
There is also a DNS-HOWTO available.
If you are running a small LAN that can access the Internet through you Linux PC (using IP Masquerade for example), it is probably a good idea to run a local name server (with a forwarders directive) whilst the link is up as this will minimise the bandwidth and delays associated with name resolution.
One point of Nettiquette: ask permission from your ISP before you start using a secondary, caching only name server in your ISP's domain. Properly configured, your DNS will not cause any problems to your ISP at all, but if you get things wrong, it can cause problems.