If you are getting your network assessed for the first time I would suggest giving the pentesters complete access.
You will get a report that is comprehensive which would include all the vulnerabilities that exists in your assets.
This will give you the complete understanding to plan upgrading the security of your network.
Moreover, during a pentest, the testers attempt a large number of attacks within a very short time. Please remember that in practice, an attacker might try these attacks slowly, over a longer period of time, and slip beneath the radar. To enable the testers to test efficiently, white-listing the testers is a essential.