Ideally, you're shooting in an environment with controlled lighting (a single light source, or several tuned to same color temperature), your subject and black or white surfaces only. In this case, the angle does not matter - just take care that its exposure falls somewhere in the middle in your test shot (so you're not accidentally clipping a channel).
In the wild, the angle becomes significant, as it determines where the card gets its light from. E.g. same card held in same place tilted towards sun, towards blue sky or towards green grass will give three different white balances. So in such case, you should hold it roughly in the same place and on the same plane as the most important surface you need to have neutral colors - e.g. middle of face in a portrait.