Mastery is an odd little beast for bears. Why? It has a softcap, but what that softcap is will vary between every single fight. It is entirely dependent upon incoming damage. Why does it have a softcap? Vengeance doesn't gain when damage is absorbed. Since SD increases the size of our absorb, it affects the maximum vengeance you can have at any given time. For any type of progression, this shouldn't ever be an issue or a concern.
If you're not tanking progression, you should only need a comfortable amount of mastery and can then gear/reforge for more threat/dps.
Reforging is fairly simple. Reforge the least wanted stats into Dodge Rating.
Yes, you read that correctly. For the first tier of raiding, we are balanced around reforging to dodge instead of mastery on the vast majority of pieces. This is a second reason why Mastery is so important to have on gear, since we won't be reforging to it.