If I write to multiple registers to configure the DDS, is it required to send an I/O_UPDATE for each register addressed?
No. It is not necessary to send an I/O_UPDATE for each register addressed in a multiple register write sequence. That said, you could wait and send one I/O_UPDATE after the multi-register write sequence to update all registers at the same time. Note, the contents of each register newly programmed rest in a buffered register. Each buffered register has a corresponding active register. It takes an IO_UPDATE to move the inactive data to the active register. In read back mode, the active register is read back, not the buffered register.