Should all resistances in a RC filter be regarded?
The time constant of the RC in your sketch is C*(R1+R2).

However, it will only do any low pass filtering with that time constant if you either ...

a) drive it at the circle with a current source, or 

b) separate R2 from ground, and drive R2 with a voltage source with respect to ground

If you drive it at the circle with a series resistor, you will need to take into account the parallel combination of that with R1+R2 as well.