According to the frost DPS thread on EJ (
here), if you are not melee hit capped, hit is the stronger DPS stats. Since Frost DPS and tank use mostly the same skills and DK threat is mostly only DPS (except for RS and DnD), I am guessing it's true for threat too as a DK Tank.
So I would cap your hit before puting any str.
After the hit cap, it seem than str is the stronger DPS/threat value, but being expertise soft capped wouldnt hurt either and, like satorri said, it will make your rotation smoother; same for the spell hit cap: missing an HB sucks with the 10sec timer, especially in AOE (in single you can replace it with an IT if you miss)