Like Phillip said one should always optimize your models for a specific goal. That said, i want to add some extra info on the creation of 3D models. Nowadays the polygons are not the bottleneck but texture space is. Since we bake so much into the texture for details (think: Diffuse, normal, occlusion, light, etc) and a top notch graphics card can only store 1GB of textures.
When an 3D object is close to the view it can takes up most of the screen which can be perhaps
1920x1280 pixels. If that single object has a
1024x1024 texture that texture is only partially seen since you only see a part of the object. So maybe
512x512 of that texture is actually seen, this gets translated to you screen resolution and thus blurred. 4
1024x1024 textures for a single object (different map types) can easily take up
10MB. Where someone rather uses a
2048x2048 texture space or larger for this object but that will take up about
40MB which would already be 4% from the
GDDR memory of a graphics card with 1GB of memory. And that just for a smallish object in a scene of thousands. Besides this map and CG designers have plenty of tricks up there sleeves for reusing objects and textures.
Still, less polygons means better performance. And since we can do so much with texture space 3D artists focus on contour (the outline of the model). It's not possible to make the outline of a model more detailed without adding more polygons, within a model we can use textures to make just a couple of polygons look as detailed as thousands.