At our company (OS) we use variable CIs for every single flight. They are printed on the OFP and our flight planning software considers up to time information for its calculation, including a second CI value, if fuel eco is positive and thrutankage could be applied.
The "optimum delay" function of the software calculates a number of combinations regarding routings, altitude, CIs, connecting pax and so on, considering also different ATC fees, AWY restrictions and of course current WX. But additional costs due to different crew salary combinations (old capt & old F/O or young capt & old F/O or ....) and posibly crew overtime are not taken into account for the time being (but will be in the future).
As fuel prices are generally very high right now and especially at our homebase Vienna, we (Fokker 70) usually have quite low CIs, such as 11 or 14.
As all of our CIs are calculated upon preflight assumptions, a second system - the flight priorities - was introduced for inflight manipulation of cruise system if becoming necessary (Pri 1: always fly high speed, Pri 2: if late adjust speed/CI to be right on time, Pri 3: if late adjust speed/CI to be max 10 min behind, Pri 4: stick to CI even with delay greater 10 min). The flight priorities are published by our network management every half year.