it does both clipping. You can define the clipping planes using the glu functions.
Whether you do your own clipping depends on your application. You can do more in addition to OpenGL. You probably
More important is culling though - you probably want to do your own version of that to avoid making OpenGL process all the triangles in your scene. This really depends on the size of the scene. So rendering methods only draw the necessary polygons anyway.
~~~ Cheers! Brett Porter PortaLib3D : A portable 3D game/demo libary for OpenGL