What is the best way in SketchUp to align the bottoms of all objects to the x-y origin plane?
I would like a better answer involving automatic alignment, but this was my solution.

Make the object a single component.
Make sure the guide points are accurately located.
Cut and copy the first component to the origin.
Copy the component to the guide points.

What also worked better was to break the grouping of components into building blocks that could be put together to make the whole.  Once the building block sliced correctly, it could be made into a component.
Note: the more complex the first layer, the more this accuracy seems to be an issue.