Hi Slaven,
During setup and upgrade you need a user with permission to create tables and such so at that time you generally should use a user with dbo permission.
However, you can harden the security of a mojoportal installation on MS SQL after installation. Since the MS SQL version uses stored procedures for everything, in my opinion the best security can be achieved by creating a new sql user and grant only exec permission for all the stored procs. Now this user can't directly access any tables and cannot do any ad hoc queries. There will be some maintenance because after upgrades you will have to give the user exec permission on any new procs in each new version.
I don't recommend using system accounts like NETWORK SERVICE for the db connection string. If you want to use a windows user, create a new user just for this purpose with minimal permission. Myself I generally don't use windows authentication with MS SQL, I configure it for mixed mode and use sql users and sql authentication.
Best,
Joe