No. The shared IP does not have to be the IP of your nameservers.
To keep everything seperate:
NS1 - 110
NS2 - 111
SHARED - 112
Now if your clients *WANT* their own nameservers they can go to their domain registar and add the host records for:
ns1.mydomain.com - 110
ns2.mydomain.com - 111
you *CAN* alternatively put NS1 / NS2 on other IPS if you want your shared to be 111, for example:
NS1 - 110
NS2 - 112
SHARED - 111
then the nameservers for clients own domains just changes to:
ns1.mydomain.com - 110
ns2.mydomain.com - 112
If you want it can even be setup using only 2 IP addresses:
NS1 - 110
NS2 - 111
SHARED - 110 *OR* 111
all methods are 100% ok, i prefer to use one of the top 2 as it keeps everything seperate, and helps with DNS problems you may come across later on.
Also, Please take a look at my server checklist in the general discussion forum. It has a few guides with DNS and giving your clients their own DNS without using seperate nameservers (as you mentioned earlier, although there is more to it
)
Chris