My experience, coming from 2 DSG Seat and current car Geartronic Volvo V40 all with manual handbrake. When I paked in my usual spot at home, I know the place its completely flat, eg...even if I put the car in park, let go brakes, it doesn't move...I don't pull the handbrake. If I park it elsewhere, if I let go of the brake after P, and it rolls till the transmission stops it moving, I pull the handbrake. When I'm on an obvious hill I would right away pull the handbrake even before I let go the foot brake, and 'curb' my wheel in the direction of the pavement (habit of driving in San Francisco, it's by law you need to do this), so if either the brakes go, it will firstly be stopped by the car being unable to mount the pavement, else, it would just end up sideway and stuck rather than hitting obstaces or people rolling down the hill.