It makes a big difference, because even a tiny bit of water will dissolve quite a bit of many drugs, including amphetamine/methamphetamine, and cocaine. Even getting 99% pure anhydrous, reagent-grade acetone, it will absorb moisture from the air after being opened, and may have absorbed moisture even before being opened. Even 1mL of water will dissolve 2 entire grams of cocaine. Not sure about meth but I know a lot of it will dissolve in a tiny bit of water, it's probably similar. 1mL of water is a very small amount, so even traces of it will cause you to lose a lot of product. That's why the drying with dehydrated epsom salts is necessary. Epsom salt will also absorb water from the air, so you have to dry it for 2 hours in the oven at lowish temperature first, too.
But it's easy, get a bottle of lab-grade 99% anhydrous acetone, bake a good quantity of epsom salts in the oven for 2 hours, and then pour in as much epsom salt as will fit in the bottle, and swirl it around a bit. Then cap the bottle. You only have to do it the one time, after that the epsom salt will stay in there and if you did it right and used a lot, the bottle of acetone will remain "dry" indefinitely.