It depends what the impurities are too. Once I was agonising over whether to buy 96% or 98.5% sodium sulphite. I asked the suppliers what the impurities were, and it was mostly sodium sulphate, which is not going to do anything noticeable in a developer or a fixer or a wash aid. The problem then was simple, since the price differential was significant.

What sort of impurities would be bad news for a developer? Insoluble stuff will settle, if you have time. Metal ions? What else?