The age old hangar/internet debate. I believe the AIM says to acknowledge the request. As was said, pushing the button indicates an acknowledgement. Saying, "IDENT'ing" is also an acknowledgement, but usually unnecessary.
For work, I have an IDENT button on my cyclic, which is nice and I can push it before ATC finishes the radio call. That assumes I can remember where it is, as it's in the same place as the AUTO-DEPART in the -60 and my brain still takes a second to calibrate.
I'm going to argue this one. It's not uncommon for someone to come up on Center, Approach, or even a tower freq and not be in the right place. ATC might know who they are, but the person in the plane might not be in the right place. See also: some of the septuagenarians and octogenarians that fly around north Florida.
Plus, it's not like ATC hasn't given wrong freqs before.
I will say if I'm not flying "local," I'll just say "Center" a lot of times because I don't always catch who I'm being kicked over to next.
Technically it's "in sight" per the FAA. But I've noticed controllers who work fleet concentration areas are far more tolerant of "Visual," though.