Why do you recommend AC coupling, rather than DC coupling, at the clock inputs?

AC coupling is recommended for the clock inputs on the AD951x parts because these inputs are internally self-biased. By AC coupling the input signal, there is no need to match the common mode voltage of the inputs, and there is no need to set the common mode voltage of the inputs. Only the voltage swing (amplitude) of the signal must be considered, lest the input range be exceeded.

DC coupling is possible because the self-biasing internal network has an impedance of about 1.5k ohms. As long as the source common-mode voltage is within the common-mode range of the input, it will work.