What is the best practice related to using the Default Update Set for moving customizations between instances?
The answer would be true. Because each time when you use a default update set for customisation. Once your development has done. you need to move that Update set to Test Instance. In this case you have to close the default Update set. And servicenow creates another default1 update set when you close the default update set. It is also not a good practice to use Default update set.
Best practice would be to use a update set with proper naming convention
https://www.servicenow.com/community/developer-forum/best-practice-never-assign-a-default-update-set-as-an-update-set/m-p/2159061