The inductance of a 3 inch long 26AWG wire is 1.23µH. It plays havoc with the diff probe if not properly accounted for. I ginned up a simple circuit to help me visualize how to dampen the ring associated with the lead:
With 620Ω, the response to a 1V input step with a 3.5ns risetime is:
I'll be making couple of sets of custom leads, with unique values of series resistance. There is no need to add any resistance to the PCB. The lead set to use will depend upon the characteristics of the signal to measure.