From the airbus "getting to grips with..." series:
As it can be seen, it is not really advantageous to fly at very low cost indices as fuel savings are not significant compared to time loss. Although using slightly higher fuel, a slightly higher cost index gives significant time gains.
For instance, for the A319, increasing the cost index from 0 to 20 reduces
the block time by 15 minutes (5%) for a fuel burn increase of only 200kg (2%) on a 2000nm sector.
Is 15 minutes of maintenance time (and crew wages, if your crew are paid on flying time) worth the cost of 200kg of fuel?