In one's own writing one can be a style manual to oneself. Nonetheless, always capitalizing "white" and "black," when used as racial terms, is not a stylistic norm. Doing so on a large scale could make it seem that the terms have a formal, permanent acceptance that many, including many in the sciences, do not grant them.

If I were to begin always capitalizing "white," I might appear to some to be a white supremacist.

I do not presume that every agency or firm that asks for racial or ethnic self-identification is predisposed to discriminate. But when they all claim that they do not discriminate, yet still ask for the self-identification, what is one to think they're doing with the information?