pcr - When designing primers how important is the GC clamp?

I will offer my own (empirical) account of primer design. A GC-clamp aids in specificity of the priming and therefore contributes to the overall efficiency of the PCR reaction. In the past, I have (out of necessity) designed primers that had both too much GC-clamping on the 3' end, and used primers without any GC-clamping. These PCR reactions were performed successfully, with differing levels of primer-dimer formation and overall efficiency.

In my experience, GC-clamping is nice, but not strictly required for good PCR. My general rule-of-thumb is to terminate my primers with 2 G/C wherever possible. If something is going to fail, it is not usually the PCR.