There is a lot of information regarding building to control moisture such that mold/mildew doesn't grow. But is proactive use of mildewcides (eg borates) a wise additional precaution? For example, on exterior sheathing as Huber says is OK? Or under crawlspace plastic or behind basement insulation? In very humid areas, my experience has been that some mold smell in houses is common (ie, I don't buy into "don't worry about it, it's harmless").

I understand that cellulose insulation and most paints contain mildewcides and that they are beneficial. So why not elsewhere, before construction is complete?

I know several people who got quite sick, apparently due to mold mycotoxins.

There is at least vendor of treated lumber out there, but I can't say it's running away with the market:

http://www.conradfp.com/building-products-bluwood.php

Thing is, it's possible to build with sufficient moisture control to not really need the full-on chemical treatment solution to the mold problem.

Application at the site during construction is probably much more cost effective.

Say for instance one's basement seems reasonably dry and XPS is added to the walls. Then there is an unusual amount of rain and mold grows between the wall and the XPS. With air sealing never being perfect, the odor is released into the basement. Sure, the basement wall drainage and water/damp proofing should have been so good that walls are always dry, but large numbers of basements somehow don't meet this standard.

With crawl spaces, I expect the standard is to just let mold grow behind the plastic but negative pressurization prevents leaking odor from entering the house. No mold would open up some other options.

Termite protection is another benefit of borates.