When I flew for a living it was company policy, written into the Ops Manual, that you did not cancel IFR, you flew the procedure every time, even in CAVOK. That was based on a long history of people changing their plans at the last minute and CFITing as a result, or making an approach to the wrong runway, or even street lights (the literature is full of such incidents.)
Our SOP was to fly the needles to minima and only then look up, every flight. That was partially to ensure that we remained utterly current at all times, even during long Summer months.
To say that you cannot, should not or must not fly an IFR procedure in VMC is palpable nonsense, in Class A, D or G.
Obviously if you are flying a procedure into a visual circuit you have to look out, particularly for reported traffic, but the main source of navigation information remains the instrument panel.