I would say currently never reforge parry/dodge or mastery. The ideal to aim for is a combined 75% between the three. What you can do is reforge Hit and Expertise to these stats to level them out and raise them more. Currently, threat generation due to Vengance just isn't that much of any issue if you have a solid grasp on the priorty for skill usage and smart group play.
I myself am sitting at 3.64% hit and 3 expertise and have no issues with threat and just hit a little over 70% of my combined parry/dodge and mastery. I have no issues with threat when DPS follow marks and play smart, and even if they don't its usually nothing a taunt and a few seconds on their target doesn't fix.