Is there a convention to use (0, 0, 0) as the center of a CAD drawing? If so, why? Wouldn't it be easier to use (0, 0, 0) as the bottom left corner and start drawing on the positive coordinates?
While there are no conventions for the cad file itself, most folks will look to use the coordinate system datums to their advantage. This involves considering the fit and function of the part.
For example if a block is bolted, consider using the bolt hole rather than the edge. However if the two edges of a block have a bigger impact on how it fits, consider using their connecting corner.
This can also reduce time required to recalculate constrained positions in an assembly when parametric parts are modified. Since the constrained features will not move as much, smaller transformations are performed on the part's coordinates as a whole and it tends to make convergence to meeting constraints easier.