Though it does not add weight to the camera, it is for sure adds weight in front and above the gimbal and therefore needs to be compensated with the placement of the camera and monitor/battery system.
Basically you mount everything above the gimbal as close as possible to the post. Also accessories on the camera should be as close as possible to the camera and near the post.
The Preston is in normally fixed on its place and should be considered part of the rig once mounted. The motors however are more variable in weight, size and placing.
Then with the placement of your monitor and batteries and the full camera set you can adjust and find the static and dynamic balance..... sounds simple but every rig is different and there is not really a rule of thumb, I think.
But the one rule of thumb you could find is that dynamic balance is often reached when the CG of the camera set is approx. 1 to 2 inch behind the center of the post.