Once your happy with your model smooth it for your final render, thats about it, keep it low till your happy with it then smooth.
If you were concerned with your PC performance (which your not) then you could smooth the normals, heck, if the smooth dosnet smooth as much as you want then try softening the normals.
As for when to smooth, it all really depends on if you need to smooth it for the final render.
Also it might be worth noting as long as you dont delete the history you can always undo the smooth.
"No pressure, no diamonds" Thomas Carlyle