Do I need to place any additional component(s) getween isolated grounds?
No resistors are necessary, however it's usually necessary to add a capacitor from input to output to conduct switching noise and keep the output from flailing around with respect to the input and/or earth. 

The capacitor has to be safety rated for the fully isolation requirements of the supply- so "Y" rated typically, but there may be other standards that are applicable. Remember it's in parallel with the isolation provided by the insulation in the transformer, PCB and opto (if you have one). Failure of any of those isolation barriers could result in an unsafe condition, so the standards are very high (high voltage rating compared to mains voltage and very, very low tolerable failure rate at temperature extremes).