If you sepia tone ever, do not keep your negatives in the darkroom! Other chemicals/fumes can have an adverse affect as well.
I like the fire safe idea, but I have upwards of 40 binders of negatives now, too many to keep in all but a really huge fire safe... Temperature fluctuations like you mention are much less of a concern than humidity. If you live in humid conditions, consider a dehumidifier or storage with desiccants.
FWIW, I keep my negs in a closet in a bookshelf in dust-proof binders and in polypropylene pages.