When a reseller account is setup in WHM with its own nameservers it can be setup to use the shared Server IP or one of the Nameservers IPs (or any other IP for that matter) as the default shared IP for all cPanel accounts.
Having it on one of the nameserver IPs makes it easier if you ever have to move servers as all traffic can be forwarded to the new IP address on the new server in one go rather than account by account.
Need SSL Certificates? WEBYSSL.com (rapidssl.com International Affiliate)
Put Your Mark on the WWWorld with WEBY Systems