That depends on whether your a 2 hander tank or a dual wield tank, because I believe 2 hander tanks need about 8% (232 or something like that) hit rating to be effective and dual wielding needs a lot more so again its all about what your using.
Just go 8 percent (7 if you are a Dranei or can rely on having a dranei in your party). This will cap you for melee specials both DW and 2H. More hit for DW will just increase white damage, and probably isn't the most effective place to be spending item points (30 expertise is however a good goal for DW).
Hit cap (max) for 2H is 8% including talents. (Except Dark Command)
Assuming you're Frost DW, you can nip 3% from talents, requiring only 5% hit before you max out yellow hit, but you can still benefit from more hit due to white swing penalties.
That being said, you don't really NEED these caps, you should rather focus on Expertise first, especially as dual wielder. 30 expertise seems to be a good goal for most DKs.