I always have my hit either at cap, or above yet I NEVER gem for it. I suggest try doing ICC rep grind for the tanking ring which has hit, or doing ToC 25 for trophies and get the pants which gives 1.62% hit. IMO, hit is an important factor, but not important enough to gem for it. So just get it from gear when you can.
I myself don't replace my hit gear if it drops more then 1%, but depends on what you really want. If you're a main tank though, I'd say always try to say at 8% or more unless your gear is absolutely fantastic, or you're not to far below the cap, because threat can be an issue if you have really good dps who are aggro whores.