Here is a little trick for how to set default view mode (flat or hierarchical) on customer list.
First, mouse right click on customer list and select the view mode you like, I select flat mode for example.
Then, again right click in customer list, and select "customize columns", add or remove any type of columns you choose.
Now my default view mode is set on "flat view mode"
FYI, I'm using quickbooks 2015.