Sorry I'm late to the party. Setting the Origin up at the center of part is more than convenient, it's almost mandatory. But not because it's "an Origin", but because the Origin establishes the location of 3 planes (X, Y and Z). Mirroring and other features are all driven by planes, and Datum Planes are the secret to using SolidWorks. If you work off surfaces, you'll be sorry because at some point the feature's surface may be edited to a larger or smaller dimension, which may change the part geometry, thus driving massive additional editing. Additionally, holes and round protrusions are almost universally used for mating to adjoining parts. If multiple Datum Planes go through the center of a circular feature (male or female) then more types of Assembly Mates are offered during Assembly. So before you ever start the Part, you almost need to ask yourself, "How will I join this Part to the next one in Assembly mode ?" Hope this makes sense.