The right number of reduces seems to be 0.95 or 1.75 multiplied by (<no. of nodes> * mapreduce.tasktracker.reduce.tasks.maximum).
With 0.95 all of the reduces can launch immediately and start transfering map outputs as
the maps finish. With 1.75 the faster nodes will finish their first round of reduces and launch a second wave of reduces doing a much better job of load balancing. Increasing the number of reduces increases the framework overhead, but increases load balancing and lowers the cost of failures.