Stat Priority as Arms:
1. Hit Rating up to 264
2. Crit up to 35% in battle stance
3. Balance of STR/Crit and work on Expertise capping
Assuming you are hit capped, because if you're not, you should gem for +hit until you are [Rigid, Yellow gems give straight +hit]. Otherwise, use the following mentality.
If a socketted piece of gear does not have a significant set bonus, socket all +crit yellow gems in that item. If you want the socket bonus, or find that you would benefit more from hybrid gemming it, then go for the socket bonus. All "prismatic" sockets, or generic sockets should be gemmed with +Crit also.