I agree with varying the stiffness parameter to get a range of design values.
If getting guidance from a geotech make sure that they are considering all of the soil layers that will be affected by the stress bulb under the mat foundation. I've had goetechs provide a subgrade modulus based on a plate load test only, when questions on this they said they thought the structural analysis software would somehow take account of the different soil layers and the stresses in each!
If you dont have info from a geotech, the attached is from Design of ground-supported concrete industrial floor slabs by British Cement Association and gives some typical values for different soils.
Depending on how the loading on the slab, varying the stiffness can have a significant effect on the forces in the mat.
The stiffness of the support to the mat should also be varied to account for the soil continuing past the footprint of the mat (stiffest in the corners, less stiff along the edges and less stiff again in the middle). E.g. if you have a raft foundation with a uniform load supported on constant stiffness winkler springs, you will not get any bending forces in the slab in analysis software. In reality the stresses in the soil around the perimeter will be lower as the load can spread out through the soil beyond the footprint of the raft, thus the raft will dish in the middle as the soil around the perimeter wont settle as much and bending stresses will develop. There is a publication by IStructE - Soil-Structure interaction, which has a couple of examples which show how using the simplistic winkler spring model can give completely the wrong result (bending moment causing tension in one face, whereas in reality the tension would be in the opposite face!) depending on the stiffness of the raft, supports and loading arrangement.
