I would suggest you put it somewhere intuitive to you. If you prefer to keep all DNS entry templates under the DNS service templates, this will mean that all clients that utilize the DNS service, will have these entries. This is important because you can enter the entry templates under any service, which means if only a portion of your clients use (say) FTP, then putting a DNS template under FTP makes sense because then only those clients using FTP will get the DNS entry corresponding to FTP services. Since all clients are going to need access to the control panel, it would make sense to put it under system or DNS as these will get allocated to all clients (assuming).
You can also create DNS templates per hosting plan, which is useful if you want to organize your DNS entries at a higher level. Basically it all comes down to how well organized you want your system to be because it will only get more complicated as it becomes populated with clients and creating a clean and intuitive interface now will help you in the long run.
Further clarification?
Jack