its some how too important for the business to have some customer fields to be mandatory to be filled .
is it in the plan to have an option that allow to select some fields to be mandatory fields?
Manager will not impose restriction to create an invoice if particular fields do not have any data in them. it is the responsibility of the user to fill whatever data is needed.
if the fields are mandatory then you can set them as custom fields for your Customer and enter individual data for each customer or enter details as a default for all customers using Form Defaults. this will automatically show on all invoices when the customer is selected. please read the relevant guides.
@sharpdrivetek makes good points. Manager is designed to minimize accounting errors. But it deliberately avoids enforcing policies. That is up to you.