If you are IFR and have a clearance to take you to where you are going why cancel,, If you are in B and wish to operate VFR ask for the visual if landing or VFR on top direct to ,,, if transitioning. You typically save very little time or distance either way and don't have to worry about it. If you are outside and you cancel before entering you need to hear you are cleared into class B as your clearance is terminated when you cancel and VFR you need to be cleared to enter,, always. And never assume anything, If you have doubt ask prior to breaking a reg.