Very old thread and a lot has changed since then. Let me try to respond the best way I can.
With the release of the 3D viewer (3D Viewer is fairly new functionality), oversized PLACE_BOUND_TOP can make the component view in 3D very misleading, even if the PLACE_BOUND_TOP was sized the exact size of the component. Obviously this is greatly improved with the ability to map a true 3D STEP file to the components so the 3D View is very realistic.
IDF Export to Mechanical Package:
You do have the ability to remap a components 3D representation (3D Box) using a different Package Geometry subclass when exporting an IDF file to a mechanical package but it does not change the way the 3D viewer looks in Allegro. The variables that control this is IDF_PLACE_BOUNDS_TOP and IDF_PLACE_BOARDS_BOTTOM with a value matching the SUBCLASS name under Package Geometry. During export it will look at these subclasses for shapes to drive the 3D Box instead of the PLACE_BOUND_TOP and PLACE_BOUND_BOTTOM but it doesn't help you with the 3D Viewer.
Component DRC Errors:
The PLACE_BOUND_TOP and PLACE_BOUND_BOTTOM Subclasses will generate a DRC when they come in contact with each other during placement which is one of its main purposes. You could make it smaller to just include the body of the component but you may miss a component to component DRC because of it. Sure, you may see a Pin to Pin DRC when the pins come in contact with each other but you will not see a DRC on the PACKAGE_TOP and PACKAGE_BOTTOM subclasses.
PACKAGE KEEPOUT defined in symbols:
You have the ability to define a shape on PACKAGE KEEPOUT / TOP in the symbol but when these shapes come in contact with each other during placement they will never show a DRC Error. A DRC error will only occur when a shape from PACKAGE GEOMETRY / PLACE_BOUND_TOP comes in contact with a shape on PACKAGE KEEPOUT / TOP.
I really don't have an answer for you regarding the results in the 3D Viewer when the PLACE_BOUND_TOP is defined larger than the component but if the PLACE_BOUND_TOP does not include the pins of the device you may miss component to component DRCs during placement. The original intent for the PLACE_BOUND shapes were to provide a simple way of generating DRCs when two component come in contact with each other.
The other discussion in the thread was the DFA_BOUND_TOP and DFA_BOUND_BOTTOM subclasses, they are used by the real-time DFA checks that are built into Allegro. You develop a spreadsheet of component to component clearances and they will be checked to the DFA_BOUND subclasses.
Sorry for the long response
Hope this helps,