Answer: The resolution of the A/D or the D/A is strictly dependent on the needs of the application, not on the resolution of another IP in the bus.
In generating a D/A signal, if the device being controlled is not able to differentiate the small voltage change of a 16-bit D/A output, then there is no advantage in using a more expensive 16-bit analog output board. An analog output board with a 0-10 volt output and a 16-bit A/D converter is able to make voltage changes as small as .000152 volts.
16 bits of resolution = FFFFh = 65536 discrete changes
10 volts / 65536 = .000152 volt per 1 bit of change.
If you are trying to control a mechanical device, such as a valve, it is unlikely the valve's position would be able to change in this small of an increment. In this case, the 12-bit D/A is a better fit.
12 bits of resolution = 0FFFh = 4096 discrete changes
10 volts / 4096 = .0024 volts per 1 bit of change.
For the same reason, there is no advantage in selecting 16-bit resolution for your A/D if the signal to be measured is of low quality. If the signal you are measuring contains a high level of noise or is unable to differentiate small changes (e.g. a thermocouple or RTD sensor) then measuring at the higher resolution would not produce additional useful information.