When you want a complete mesh for texturing (and if the object your modeling is one complete surface) then merge, if its a collection of parts then group, for example a guitar is a collection of parts, body, neck, pickups, dials, strings etc etc, so I would model the each part sepperatly amd then group them. but I would keep the bodfy as one mesh )i.e not have the front of the guitar sepperate to the sides and the back of the body.
Another example would be a head, I would keep the face as one mesh and then have the eyes grouped to the head, then you can texture and animate them sepperatly (pretty obvious in this case).
hope this makes sense.
"No pressure, no diamonds" Thomas Carlyle