At a certain gear lvl, you should start gemming crit. From what ive read, a mid to high 480s ilvl is where you would probably gain dps from gemming crit. It supposedly sims higher at that point. The only exception is if you are a jewelcrafter. You still want to use the strength JC only gems if you are arms.
Every sim I've run has haste>mastery after around 490 ilvl HOWEVER it's always been incredibly close, like .03 SEP. Since mastery is better for fury, I just stick with mastery reforges for when I play fury.
My last sim ran crit at .55 SEP. All this really means is that most times you gem pure crit, unless you can get a socket bonus (which are worth reaching for arms in most cases, but often aren't worth getting for fury.)