For any real neuronal morphology the soma electrically extremely compact--it is essentially isopotential for all frequencies at which the cable equation is relevant.
You can run a simple test to prove this to yourself. Create a model cell in which the soma's morphology is specified using the pt3d syntax, e.g. a soma that is 20 um long, 15 um wide at the 0 end, 2 um wide at the 1 end. Attach an "apical" dendrite to the soma's 1 end--make it 2 um wide and maybe 1000 um long. Use the d_lambda rule to discretize the dendrite. Set the soma's nseg to 25, make all membrane passive, and set Ra to 100 ohm cm in all sections. Attach an SEClamp to soma(0.5) and apply a 50 mV step command. Plot V vs. t at soma 0.5 and at the apical dendrite's 0.5 location. Next change soma's nseg to 1 and repeat the simulation. What difference do you see?
Then return soma nseg to 25, remove the SEClamp, and attach an AlphaSynapse with tau = 2 ms, e = 0 mV, to the dendrite at about 100 um from the soma. Adjust the synapse's peak conductance to elicit a 1 mV epsp at the soma. Plot V vs. t at soma 0.5 and at the apical dendrite's 0.5 location. Next change soma's nseg to 1 and repeat the simulation. What difference do you see?