I think you should either use motion blur all the time or not at all. Motion blur gives the overall animation a different look, and if you have it enabled in some sequences and disabled in others, there is the danger that these sequences don't match from the overall visual appearance. Rendering with a 2D oversampling motion blur like C4D's scene motion blur or ProPack's motion blur will make everything render longer, as it is rendering the whole scene multiple times, no matter if it's moving or not. If you use the 3D motion blur of FireFly or similar implementations (e.g. in PRMan or 3Delight), the render time will depend on the motion - objects that move fast take longer to render, objects that move slower or don't move render only slightly slower than without motion blur.