Yes. If you’ve ever added extra permissions to the users of your additional lines, you must ensure you reset them.
You should set the permission to Read only if you want full restriction of the users on your account after opting them in to My Account Controls.
This will mean they won’t be able to change their own permissions and they won’t be able to purchase data or Extras.
If you’re not sure, or can’t remember if you’ve ever changed permission levels, you can check these by speaking to an advisor on Live Chat/Retail/191 online or by speaking to an adviser on Live Chat.