What's best practice for setting up client/agency ownership within Google Tag Manager (GTM)?

1) Setup a new Tag Manager "Account" for the client or even better have the client set up their account and add you as an admin, 2) create 1 x GTM container for the client in that Account and 3) then install that GTM container on the client's website.

This way it's the client's data and configuration, which should remain with them regardless of whether you have admin access or not.

Please don't house client accounts in a master agency account. While you can give them container access in your main account, their use of GTM will always be tied to you, even if your relationship ends. This gets more difficult to unwind over time. 

Learn more and enroll in our Google Tag Manager Course.