It would depend on the design. If it's a combat model or a stunt model with little or no fin area then obviously something would need to be added. If it's got a sizeable fin area already then it might very well be just fine.

For example a Flite Streak would likely be fine with no added area to the vertical fin. Just put a hinge line at about 50% chord. A Ringmaster might be OK or it might end up doing some funky things in turns and at lower airspeeds. It would certainly be in the marginal range. There are a number of stunt models that have no fin. Just a deepened rear ridge line to the fuselage. On such as those you'd want to add some sort of rear fin.