The problem with trying to use "white paper" as a neutral color reference is that ordinary white paper is not necessarily truly neutral. It is far better to use a product which is guaranteed to be neutral.
A neutral gray card (such as the WhiBal product, for example) is far more versatile than a white neutral reference card. Why? It's because it can be used for both a reference in setting up a "custom white balance" in the camera and it can be used as a reference in the scene. If you put a white card in the scene as a reference, it could be very easily overexposed, blowing out at least one color channel and thus becoming essentially useless.
Since I always shoot in RAW mode when the color of my images is important (which, for me, is all the time), I merely place a WhiBal card (or any other calibrated neutral gray card) into the scene for a test shot. Then, during RAW conversion (a post-processing step), I use an "eyedropper" tool to sample the gray card in the test image. The resulting color temperature and tint numbers are then transferred into the RAW conversion of the rest of the images in the series (in a single batch conversion of the rest).