IS 456:2000 does talk about the stiffness of members in Clause 22.3? However, it does not provide the modifier values to be used. You can refer Code Section 10.11 of ACI 318 for stiffness modifier values suggest by ACI.
Stiffness modifiers can have a significant effect on the behavior of the structure. In absence of stiffness modifiers, the structure would be stiffer and thus attract higher lateral forces due to the earthquake. So you might end up with heavily reinforced shear walls, moment frames etc. At the same time, you might also underestimate the drift. Stiffness modifiers will also affect force distribution among different members of the structure.