Hi, there
   As we all know LNA should have a specific input impedance for matching,  but mixer and all other block dont have that specification(in RFIC design). I wander if I can explain it in the following way:
    if the signal is transmitted in the form of wave (e.g. a LC network.) and the wavelength is much smaller than the transmitting path, you should take the reflection into account. on the other hand, if the signal is transmitted in the form of diffusion (e.g. a RC network.) or even if the signal is transmitted in the form of wave, the wavelength is large compare to the transimitting path, the voltage is more relevant.  
  I'm not sure if I am right or wrong. Any comment on this is welcome.

Best Regards

Julian