This is always up to debate.
It's basically summed up to this.
Hit 100% of the time for 100 damage
or hit 90% of the time for 110 damage.
Now when your streaks are going good and you're hitting a lot at 110 damage, your dps will be higher, and on specific fights you'll see consistent gains. But anytime you miss, that's 0 dps being generated that second. That's why most people advocate reaching 100% hit/expertise capped. Really what you should look at it as is reducing that number to a point where the chance to miss/be dodged is low enough that a STR or ArP increase will outshine it.
I'll admit I run at 17 expertise instead of 18 as arms, but I find it hasn't negatively impacted my DPS as much as I had thought, however if I was at say 6 expertise, I'd imagine getting a string of dodges would greatly reduce my damage capabilities not to mention my rage generation.
My personal recommendation and for simpler ways to balance your gear, always achieve hit and expertise caps first, then focus on the other pieces. Just be open minded about how to achieve this, sometimes it means regemming one piece of gear, to equip a new piece of gear with less hit but more STR/ArP.