The benefit of applying more threads to optimizing a specific problem varies depending on the optimizer you use and the characteristics of the problem. You should experiment to assess performance improvements and degradation when you apply more or fewer processors.

For example, consider the following results:

Root node processing (before b&c):
  Real time             =    3.22
Parallel b&c, 2 threads:
  Real time             =    8.29
  Sync time (average)   =    2.74
  Wait time (average)   =    2.48
                          -------
Total (root+branch&cut) =   11.51 sec.

Another key consideration in setting optimizer and global thread limits is your management of overall system load.