This is a tough question. Generally, the sodium salt is the reagent of choice -- sodium salts are the cheapest, what with the planet having oceans of raw material. Sodium salts tend to be more soluble than other salts. Potassium is a bit more expensive. And it has an important use -- plants need it to grow, so we tend to reserve it for those applications. Or other reasons it may have slightly different properties. For example, sodium salts are all very hygroscopic. Maybe since potassium permanganate is used as a standard, we want a salt that's easily dried, and easily kept dry. This may have been a more significant a century ago, and we're just stuck with it now. Lithium salts are pretty rare, and lithium has human physiological effects -- we use it for medicine, not reactions.