If you're in a group that's unfortunate enough to not have a Fury Warrior, Feral Druid, or Elemental Shaman, then the minimum amount of crit you'll have is 15%. In that case, the break-even point where Mastery = Crit is when you have 18.75% mastery if you have 15% crit. Even if you favor crit, you probably shouldn't have less an equal percent of mastery. So with 15% crit, you should aim for 15% Mastery as the
minimum. For most players that still requires reforging to mastery.
So in general (and I think there are very few/almost no exceptions to this rule) resto druids should be reforging to mastery.
The argument that crit is slightly better than mastery is correct, but the fact that we start out with so much crit just reduces its relative value to mastery. There's almost no way any druid's mastery is high enough to justify stacking more crit than they already have.
[Edit] To simplify my suggestion:
If you have Nature's Majesty (4% crit balance talent)
- And the 5% Crit buff*: Stack mastery until you have between 22% to 25% mastery.
- Without the 5% Crit buff: Stack mastery until you have between 17% to 19% mastery.
If you don't have Nature's Majesty: Stop being bad and get it.
*Note: The 5% Crit buff comes from Leader of the Pack (Feral Druid), Rampage (Fury Warrior) or Elemental Oath (Elemental Shaman)