About the use of the handmike:
Supposing the PNF does the comms:
During manual flight (very often the last 3000 ft or so), it is rather annoying for the PF, if the PNF uses the toggle switch on the yoke. It is also quite awkward for the PNF to use the toggle switch on the audi selector panel, as this is the same hand he needs to operate the gear/flaps/MCP etc.
This is the reason why I always use the handmike, with headset on. Handmike is in my left hand, and my right hand is free to operate the levers and switches. (me being in the left seat;-))
Headsets on below FL100 is the rule in our comp.