Expertise reduces your chance for something to dodge or parry you. 26-27 expertise means unless you are hit with some weird debuff like attuneman used to that mobs or bosses cannot dodge your attacks. 57 (I think) eliminates their ability to parry your attacks.
The hit rating cap for 1 hand attacks is 267, but from what I have read some attacks like shockwave require slightly more to achieve the same goal, which is mobs and bosses not ever being missed by your attacks.
After 540 defense your goal needs to be determined by the people you play with honestly. If your healers are having trouble keeping you alive, stam and avoidance is the goal. If your dps is pulling stuff off you, hit and expertise needs to be the goal.
P.S. If you have a draenai in your party you can carry less hit rating, not sure exactly how much (hordie here) but it is 1% I believe.