You should really have different sets with different values of hit rating. If you aren't capped, +hit is the single most importan stat, but if you are capped, extra +hit is totally wasted.

For Heroics you should aim for 3%, for 10 man 14% and for 25 man 11% (10% if you are alliance). 25 man requires less because you can assume you have a bala druid or shadow priest in the raid (and a draenei in the party if you are alliance) while in 10 man you can't be sure.

Edit: of course if you are a draenei you can lower by 1% also heroic and 10 man cap.