There are many ways to hold poi, just go with what feels correct.
Personally, I have ball grips, which I hold with the tether between my thumb and forefinger.
"Wall plane" as a concept is generally parallel to the axis through both shoulders, (so in front and behind you).
On the gripping hand, if you are trying to clean up your planes by spinning next to a wall, then the idea is to have the poi graze the wall during normal spinning, so it will depend on what you are working on.
'We're all mad here. I'm mad, you're mad." [said the Cat.]
"How do you know I'm mad?" said Alice.
"You must be," said the Cat, "Or you wouldn't have come here."
- Lewis Carroll, Alice's Adventures In Wonderland