Hello, 

I know this is an old thread but I think I had the same question about making all time default to billable in QBT.  I followed the instructions to make billing required but also went to the customer's tab on the left in QBT.  Then clicked on a specific customer. Clicked on Custom Fields Tab.  Clicked on items next to billable.  Moved Yes to the assigned items tab and hit save.  Now on every time entry for that customer it will come up yes for billable automatically