Chemistry teacher here...
The rule is acid to water only. However, in the practical world, it's only important when dealing with highly concentrated acids. Heat is generated when acids and water are mixed. The sudden heat generated can cause it to boil and splatter. Pouring a little water into a strong, highly concentrated acid, the water can 'flash' boil, and spatter. Even when doing things correctly, I've melted plastic bottles when mixing strong acids and bases.
(It's best to have the bottle submerged in cold water when diluting highly concentrated strong acids... especially if you are working with larger volumes)
As for adding water or acid to the battery....
You can think of acids as solutions... like salt water. What happens if you allow salt water to evaporate? That's right, the only thing that evaporates is the water. The salt is left behind. Same thing goes with acids. As they evaporate, only the water dries up. If you keep adding acid to replace the water, the solution gets more concentrated. All you need to do, is replace the water that evaporates over time.
As for the specifics of a battery, Zigmont is right on. Do NOT store a battery in a discharged state if you want it to last. A vehicle that is only used on occasion will benefit from a Battery Tender, which is an automatic trickle charger that automatically turns off when the battery is fully charged. Chargers, even trickle chargers that do not have this feature will kill a battery by overcharging it.