Answer: The rule of thumb is that the grout should have a lower permeability than the surrounding soil so that water does not migrate through the grout to the intake area of your standpipe piezometer. The standard practice is to place a bentonite seal above the sand intake zone and then place grout above the seal. Any lag time between changes in pore-water pressure in the ground and changes in the water level in your standpipe is really from the volume of your standpipe and the permeability of the surrounding ground. The grout itself has little influence so long as it prevents migration of water from above. We sometimes recommend the same grout mixes that we use for vw piezometers.
You might also be interested in a technote on grouting-in diaphragm-type piezometers (eliminating the sand intake zone altogether). One of the references listed at the bottom is Dr Vaughn, who reports results from his experiments in grouting in standpipe piezometers.