Well, it is also important to know that the initial height of placing matters.
What i mean is, that when you place two platforms next to each other on a steep area, it's nearly impossible to bring them to the same Z-Height.
Thats why I started to use 'building areas' in my terrain. This are areas in the map which are flat and have a fixed height. Now when I want to create a building from multiple doodads or place invisible platforms, I pre-place them there increase their Z-Height at least once and move them to their final place adjusting their height there. Important to note here is that you should uncheck the 'reset Object height' in the advanced settings of your editor, or the doodad will reset its height to the ground everytime you move it.