I never put my hand on the catcher's back. I think it's unnecessary, and makes many catchers uncomfortable. Some PUs think it helps them "feel" where the catcher's moving when the ball gets away. I hate that mechanic.
Besides, you must be extra careful these days about sexual harassment. A few years ago, a PU was hauled into court when he put his hands on the back of a female catcher.
The ONLY time I put my hands on a catcher was to keep her/him from falling.
Bob