Hit is binary. 1 or 0. If you need hit, its value is very high.
Some people talk of hit as a sloping curve, approaching zero as you approach cap, but it is actuallly not like that, it is a steep cliff.
As such, until you cap, it is your most important (secondary) stat.
However, to say that you MUST hit cap is a bit of a falacy. You can DPS at 15% hit, or 14% or whatever. Will you do your maximum potential dps? No. But being under doesn't stop you from DPSing, and being so close to cap (less than 1%), means that you probably won't notice a difference in game. When you are doing 30k-40k dps in a raid, 100 dps is nothing.
There are a few situations where other stats are worth more than hit:
1. Intellect is worth more than hit.
2. Spellpower is worth more than hit.
2. Haste, as you approach a haste plateau, is worth more than hit.
So if putting on a piece of gear would get you more intellect or haste at a loss of hit, it still might be worth it to equip that piece of gear.
In general, you should always be trying to hit cap, and in general, you should sacrifice every secondary stat to get it, including haste. But be aware of your haste plateaus, especially when you are near them, and don't be afraid to drop a little hit if you have access to a higher ilevel item (with more sp or intellect).
To sum up, there is no "safe %" under hit. There's just capped or not capped, and then case by case basis for items based on their intellect, sp, or haste levels.
"Falling from heaven is not as painful as surviving the impact."Revenant
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