I have decided to bring the issue up with my CFI.
Exactly.
"Positive transfer of controls" is in the PTS, and I've seen exactly one instructor who I didn't have to say "paws off" to at some point.
I'm a stickler for that. I've seen perfectly good tires destroyed because an instructor tried to be helpful, so I insist on some statement. In an emergency, permission is not required, but some statement is, even if it's just "I got it."
The story is that I, as PIC, decided to abort a takeoff that wasn't going how I wanted. No-flap takeoff on a moderate runway in a 177RG. Rotated (probably a hair early or perhaps a bit too much), lifted off, then settled back to the runway. This was during an initial complex checkout, so an instructor was on board. He stomped on the brakes, silently. Since there wasn't much room, I decided that no-one on the brakes would certainly overrun, and two on the brakes probably would not, and I wasn't sure what he was doing, so I kept on the brakes, too. Not surprisingly, the right wheel locked up and flat-spotted badly. In retrospect, I should have told him to get off the brakes, but the whole thing only took several seconds.
During the next checkout, a year later (this time in a 182), a different instructor "helps" me position the aircraft in the run-up, and gets the lecture. The flight controls have one operator at a time. Period. And that operator is changed verbally, always. The three-step FAA thing for routine changes, and any expedient verbal alternative for an emergency.
Much to my surprise, I've had to repeat that to almost every instructor. Even my CAP check pilot, who is otherwise an excellent instructor.