I agree with Andrew... friction should only be considered as a hinderance never a help. From the standpoint of stresses due to the occasional load set I would anticipate that no friction would result in the highest displacements/stresses.

However to determine if my assumption is correct, build the model with mu, then set the mu multiplier to zero in the load case editor and set up summations for both mu and no mu, design for whichever represents the worst case.