I do CO as part of my weekly shifts, but I am currently N05. As a Photo TM, I am allowed to keep my $.50 pay differential unless I step down of my own volition to do an N03 position such as Pricing. I don't do GSA shifts since I don't want to run the front end. None of our GSAs do Cash Office at our store.
I used to be a Cash Office Specialist before they dumped the title of Specialist across the board. Now Cash Office isn't really considered skilled enough to deserve an increase just for learning it. Luckily we got to keep our $1.00 differential for that position after they cut our titles back to TM.
I would suggest applying for GSA if you want an increase to your hourly rate.