If you're removing the SD card from the bluescsi while it's on, putting more CD images on the SD card, and putting the SD card back into the BlueSCSI, yeah you'd need to reset the bluescsi with the reset button on the Pico since when the BlueSCSI boots up is when it looks for images. (If you're using the BlueSCSI as a hard drive emulator as well, probably shouldn't remove the SD card while running, not sure about if there's a particular reason not to if you're using it just for CDs)
It shouldn't matter what the power source of the BlueSCSI is though, USB or termination power