Always on my automatic. Probably a habit from driving a manual for years.
Never. I had to be reminded to do it on my driving exam. ::shrugs:: I don’t, my parents don’t and it has never occured to me to start.
Always. I drive an automatic but was taught by someone who always drove a manual.
I learned to drive in LA, and lived on a street that had a hill on it.
My boyfriend usually NEVER uses it, well, until his 1966 Mustang rolled out of the garage (twice!) when he opened the garage door in the morning. That’s precisely why, even though it’s now habit for me, I’d never consider NOT setting it. I don’t want the Golf Ball rolling out of the garage and crashing into our neighbor’s garage!
I don’t use it – but every now & then I do to make sure it works.
I do it. My first car that I owned for any period of time was a '71 Ford with a C6 automatic. Early '70s C6s were notorious for popping out of “Park” and into “Reverse” due to a design fault. Because of this I got in the habit of setting the brake and just haven’t given it up.
One of the coolest things about Cadillacs is that the parking brake release is vacuum-operated. It automatically disengages whenever you shift out of Park with the engine running and stays disengaged until you shift into Park and set the brake. Ever stop a two-ton road locomotive using the parking brake only? It’s fun. 
My previous car - allways used to but then the cable started crushing as it spanned the rear axil and would hang up which would cause the R brakes to be applied slightly for the 1st few miles till it losened up - then never. My current car uses the parking brake to ajust the rear brakes but it over tightens it so I refrain from using it unless I’m on a hill
I always set the parking brake when I’m on a slope. When I park in the garage I almost never set it.
Both of my vehicles are automatics.
I watched my dad NOT do it and subsequently watched the truck roll down the hill and go through the neighbor’s house.
I ALWAYS set the parking brake… to the point of being potentially OCD, as in going outside of my house just to check to make sure it is engaged even though my driveway is comletely flat.
Wow. I didn’t realize so many people actually used the damn thing.
When I park in a flat area, I put my car’s transmission in “park” and that’s the end of that. I’ve neverhad my car roll so much as an inch, and I didn’t think you were supposed to use the parking brake if you didn’t need it… they never mentioned it in Driver’s Ed, my father never brought it up (he doesn’t use his, either)…
If I’m parking on a slope or something where I think there’s a chance my car would be able to roll somewhere, I’ll engage the parking brake, because then it makes sense to me, but that’s it.
Always, manual and automatic transmission. Automatic always in park, manaul always in second. In all cases, front wheely turned to head the car into the curb should it manage to roll after all.