I undercoat only in white.

My army is painted red and white. Many Bret players paint each knight differently, as shown in the army book, but I personally prefer when the army is all the same color. Undercoating in white permits to avoid a lot of work, and that's good for me since I'm lazy.

not always black, but at least 85% of the time, and it is my default.

I don't use bright whites, reds or yellows very often (the main reasons to undercoat white), most of my models are dark green or brown. It is forgiving: a spot of black rarely looks out of place on a model (if you missed a spot for example), but if you can't get paint into a deep white crevasse, you're screwed. For some reason I think it makes models look more realistic too: it really suits the LOTR style of figures (perhaps because it goes well with metals, browns, greens).

Always black for me: as Swordsalot pointed out, it is a lot more forgiving, and, imo, details on the model just turn out better when undercoated black instead of white.

it depeneds for me. I have just recently under coated most of my slayers white, and then i will get the weapon and trinkets in black.

otherwise youd be daft not to undercoat dwarfs Black

Burro Boskov

I use black for something dark/mostly metallic, such as my Blazing Sun Knights, but I mostly use white. Something about yellow not working very well over black makes me do that.

Black for me. I like painting in layered highlights and the black gives me natural shading :).

I either prime Black for metal and plastics or Grey for resin models since that primer is a bit better for FW stuff and the like.

Khrangar

None, mwuhahahahahahahahahahahaha....

*cough cough*

But really, the answer is different for each of my three armies.

White for my Bretonnians.
Black for my Druchii.
Ice Blue for my 'Nids.

I always use black. I go for a lot of darker colours with Orcs & Goblins, and black suits them much better in my opinion.