i'd say white, ebcause when you're removing a background, often the shadows on the subject blend into a dark background. the only time i say a dark background is good is for a really light subject (like a blonde in a white dress XD) but generally white is just easier to work with.
I do think it depends on what custume the model is wearing. A light background for darker colours and a darker background for lighter colours is the safest I think. Dark clothes and a dark background really make it hard to distinguish between the two when you're extracting
I second that. white and grey/black both have advantages. A lot depends on the colors of the subject. As long as edges are clear enough to cut. No matter what your skill level is have a nice sharp edge is essential to stock.
HOWEVER I will say this. I'm after doing 10 pictures for a commission. The dress is ivory, and the backdrop is yes ivory. It's also overexposed but thats besides the point. The thing is to try and get some contrast between the forground and background so extraction can be "slightly" easier. Trust me! After what I just did, any extraction for me is now easy! So young manipers would find what I just did very hard, whereas someone like yourself wouldn't have a issue as we know the "tricks" to bring out the forgrounds.....
So background colours are really preferance to the model at the time.