There are currently different opinions about this. Some have claimed that Ka performance can suffer when mounted by a bumper cover (ie; fascia), while the engineers at Escort has suggested differently.
I personally have my 9500ci radar receiver on the lower part of my metallic silver painted bumper cover and am pleased with the performance I get from the 9500ci. I will however move it eventually to see if, in fact, there are any noticeable performance changes when mounting in a CLS (clear line-of-sight). X and K band apparently are not adversely affected (perhaps due to the longer wavelengths of each band respectively).
My choice of mounting behind the bumper were two-fold.
1) To see radar propagating under any vehicle that may be directly ahead of me.
2) To protect the sensitive antennae from the outside elements with something substantial (like a the front fascia).
I hope this helps you.