You can veg your plant anywhere you want! As long as it has sufficient light it will grow. The reason to do it in a cupboard would be to maximize the amount of light you are using. If you have your plant sitting on a stool in the middle of a room and a light above it, the light that is absorbed by your carpet or walls or whatever is wasted. If you had a powerful enough light it would not matter that you are wasting light. If you're just using a couple of CFL's than you want to maximize the light by putting it in an enclosed area with reflective walls.