Absolutely 
The 
sign police are always out there and in some counties they have a few crews that
collect step stake 
signs. 
Banners typically require a Permit and the counties can be very strict about that.
If your customer Paid you for the Permit, the the Original Copy Is their Property,
you should present the Permit upon completion of the installation, and the copy 
is for your records
When Code Enforcement shows up, it gets ugly quick.
Your client presents the Permit and that is that.
Did you charge a Design or Layout fee for the 
sign? If so, that is property of your client.