The problem is not always in what you bought, but in how long and under what humidity conditions it has been kept. When you open the container, if the humidity is low, some of the decahydrate is likely to become pentahydrate. Over the course of time, although the borax can still be considered to be the same purity as when you got it, the water content may not be. Or, it may be. If it's in a saturated solution, you know the equivalent weight % of the decahydrate by the temperature of the solution, but there's a lag. But if you make a saturated solution at a given temperature, let it stabilize, and decant some of the liquid, that part will have that same concentration at any higher temperature. It's really a simple thing to do and is common lab practice where accuracy is important. If your borax is pure enough, there is no need to do the purification steps, but it's still a good idea to keep the saturated solution. It is much simpler to measure out a certain liquid volume to the nearest ml than to weigh out to the nearest 0.1 g. Besides, if you really think 0.1 gram precision is required, you may not get it with a solid that has two possible states of hydration.