Follow your battery manufacturer's instructions. Wear safety glasses! For instance Trojan says:
Important Things to Remember
Do not let the plates get exposed to air. This will damage (corrode) the plates.
Do not fill the water level in the filling well to the cap. This most likely will cause the battery to overflow acid, consequently losing capacity and causing a corrosive mess.
Do not use water with a high mineral content. Use distilled or deionized water only.
CAUTION: The electrolyte is a solution of acid and water so skin contact should be avoided.