Part origins are normally located based on the geometry of the part and its symmetry. There are two general exceptions:
first is to locate the origin so that the part lines up in a layout grid at the correct position, without mates;
second is when the part is created in-context and the origin is located by projecting the assembly origin onto the Front plane of the new part.
Having the origin based on the geometry of the part is the most common method and found in most best practices documents. Creating the geometry of the part off the origin for alignment purposes is generally not done in smaller assemblies or with parts that may go into different assemblies, but can be useful in a larger assembly as the part is inserted at the assembly origin and fixed. This approach reduces the number of top level mates. In many cases, if you properly locate features of the part on the origin, your mating planes already exist. For example, by placing the absolute center of a sketch for a Wide Flange Beam on the origin, then extruding from midplane, you get a Top plane in the middle of the beam, a Right plane in the middle, and a Front plane in the middle without having to do any extra work.
When a part is created in the assembly, the origin could be at some location well away from the part geometry. This is generally unacceptable and can be fixed, or as we will see later, the in-context relationships can be created differently to avoid the origin being off the part.