Negligible for many, but possibly not for all purposes. Volume and surface resistance values for PCB substrates are given in datasheets with large margin between typical and guaranteed values. You can calculate after you have defined the test geometry completely, which istn't done in your picture (no test structure length, no substrate thickness, no specification of bottom side).
You should also consider that the actual resistance depends strongly on surface contamination, relative humidity and water absorbed by the PCB. A typical situation to get unsuficient resistance is surface condensation.