When should I model by changing the mesh of one object and when should I create another one?

I was thinking about this for some time. When I am modelling, should I try to keep the number of low and make details by changing one object by extruding, insetting etc. or is it ok if I just make some details by making an entirely new object and just snap it on where I need it (if it creates the result I am trying to achieve)? This of course only concers cases where its ok to have sharp edges.

So far I tried to make things by modelling as much details on one continuous mesh and creating new separate meshes only when absolutelly necessary (maybe not exactly necessary, more like when I dont know how I would do it inside a single mesh). So should I continue like this or should I just always pick the way that will achieve the same result in easier way? 

Is it "bad" to create scenes with a larger number of simpler meshes?

Thanks, Tomas