When designing primers how important is the GC clamp?
It's hard to provide an objective answer. If you have a decent length and good complexity, even a single terminal 3' G or C would do. Of course, one has to take into account the primer's overall GC:AT ratio and things like annealing temperature.

Here's a link to diverse opinions on the topic and it has this nugget (which I subscribe to when possible):


  FWIW, my preferred offerings to the PCR gods are primers with a single
  G or C 3', FWIW. Seems to keep 'em happy most phases of the moon.