subsurf on during render

I am currently modeling a car and I was wondering if I should be applying subsurf before I render or if it’s ok to render with it as is. The model looks like it has enough geometry to be smooth when I render it but it still looks a little blocky. If I add another layer of subsurf (but don’t apply it) it looks much smoother.
I’m getting off topic, is there a problem with rendering with a subsurf layer added but not applied. Do most people do that?

There are not too many cases where you ever need to apply subsurf. The only time I ever apply it is when I need to create a mask for part of a mesh. At that point I’ll duoplicate the mesh, select the faces that I want to get rid of, apply the subsurf, delete the unwanted verts and move the remaining mesh to a new scene where I then access it as a mask in the nodes compositor. The only reason that I apply it is because the mesh will distort into something unusable due to the catmulclark tension if you delete faces.

There’s a button in the edit buttons labeled “Set Smooth”. Use that instead of cranking your subsurf into the stratosphere. If you use the button in object mode it will smooth the entire mesh but in edit mode it will smooth only selected faces.

There’s a field on the subsurf modifier labeled “Render Level” that determines how much is applied at render time. The “Level” field determines how much is visible in the 3D window. The two are completely independent of one another but either can cause Blender to crash due to memory constraints (how much RAM you have).