If humans had 8 arms, we'd reserve one for back and butt-scratching.

So are most octopuses left handed or right handed, middle handed, or middle-right handed…

Octopodes have a different nervous system than vertebrates. We have a brain that does almost all of the "thinking" while the rest of the nervous system is either sending or receiving signals (with some "thinking" done below the brain like in the spine).

Octopuses have a brain, but much of the "thinking" is done by nerve clusters in each tenticle.

While grasping on object for humans requires signals going from hand to brain and back, for octopi, a lot of the signals can literally just stay in the tentacle.

It makes sense in that context that they would learn tasks in specific tentacles.

Same here.

I use my right hand for most tasks that require dexterity or strength.

I jerk off with my left.

I wonder if this has anything to do with muscle memory. The arm that successfully completed the task for the first time, has that task assigned to it forever.