Page 5 of the datasheet for the ADS1282 states that MCLK, M0 and M1 must be tied if unused. It does not state if they should be tied high, low or together. The ADS1282-EVM schematic shows M0 tied low, M1 tied high, and MCLK tied low. These lines also connect to other devices however, so the voltages chosen are probably chosen with respect to those device requirements.
Most likely they can all be tied high or low and there should be no problem, but a clarification on this point would be helpful.