I've been developing my system for aerial surveying, no big deal, just an Iris+ with a Canon S100, and wondering if it ever makes sense to not use GCPs even if you have RTK.
The sticking point to me seems to be in the uncertainty of camera attitude. Even if you know the position of your UAV to within 1cm, if your camera angle is off by 0.25 degrees from dead vertical, it's going to be pointing at a point 43 cm from the sub-aircraft point, in some unknown direction.
Clearly the only way for a multicopter to move forward is to alter its pitch. Pitch and roll will vary depending on the wind. Pitch and roll are logged by the UAV, but attitude is not part of geotagging of photos as far as I know. Maybe it is, and this question is moot.
But if it's not moot, then it seems like you always are going to have to have GCPs. Maybe you could get away with a single GCP?