When should I use impedence matching resistors in fast signals?
When prop. delay exceeds rise time = transmission line


effects will occur in the form of LC resonant ringing and the amount of ringing depends on the source impedance matching.
Rise Time is ~ 0.35/f for 10-90% 
speed ,v is 2x10^8 on FR4 with d=4 @1GHz [m/s]
Tp=v * x for x= path length
thus if path, x


x>20[cm/ns]/f[GHz] * 35%, you may have ringing  without good termination and controlled impedances
most people add margin and use 10% of 20cm/ns 
or >2mm per GHz or >20mm per 200MHz, return loss is affected

the same rule applies to Return Loss in Antenna >10% wavelength, although some calculate for 5% paths @ f.


For controlled impedances with a ground plane it is the track width/gap ratio that lowers Z. For co-planar it is also track width/gap ratio that lowers impedance. For track inductance it is Track length/width that determines nH/cm which is why ultra low ESL caps are 1x2 (LxW) for length between electrodes, L whereas typical caps like 603 805 1206 or 2x1(LxW) have higher ESL.


meanwhile \$ Z_o=\sqrt{L/C}\$ and velocity ratio \$v_c/c=1/\sqrt{LC}\$


for FR4  d=4.2 but @ 1GHz d~4 (depending on concentration of fiberglass) thus \$speed=c/\sqrt{d}\$ = c/2