The most appropriate x-y position of a library object is in the top left corner (x=0, y=0).

Place all the library objects in the top-left corner, independently of where you want to place them on the page when you will use them.

What happens if a library object is not in the top left corner?

If a library object is not in the top-left corner, when you place it on the page, the x-y offset as in the library window will appear also on the destination page - so the object won't be placed on the position of the pointer, but shifted for the offset in the library window.

This makes the placing of objects more difficult.

Anyway, this can help if you intend to place library objects by scripting, by means of command Page.AddLibObj(objlib, objname, x, y, 0). The specified offset will be maintained so that allows you to stack multiple library objects one next to each other or one below to each other.

About replacement of library objects not on 0,0

The replacement of library objects not on 0,0 can cause problems, as the replacement object will be shifted down and right for the offset from 0,0.

To avoid it, add to such a library object an invisible box (transparent, with no border), that starts at 0,0.