It should be done when fully charged unless the plates are exposed. If the plates are exposed just fill it enough to cover them, charge it then check and fill. The reason you do it after charge is to prevent spilling the acid out as the battery is charged.
Great advice given above, however, I do not recomend using tap water as most tap water sources contain a high mineral content that will create corrosion and shorten battery life. Good luck!
As a rule of thumb, if the water is good enough to drink then it will be fine for your batteries. If you are in an area with known heavy water then use distilled water. Proper watering and charging cycles should be the main focus.
My opinion, it makes zero sense to use anything but distilled water. For what a gallon of it costs at any pharmacy vs the cost of a new set of batteries its not worth taking the risk.
I always believed keeping the battery at its' optimum performance for maximum longevity was the main focus, so again, I would never recommend tap water. Premature plate denigration and charging failure will accelerate. I don't mean to seem contradictory to you John, but the facts are facts. Good luck in however you charge!
BTW, I have been using their products for years. I have one trolling battery that continues to give good service after 9 years... This is the tenth season on that battery! http://www.batteryde-mister.com/
Per my reps at Guest and Minn Kota, always top off before charging so the water will convert to acid. And use only distilled water. If you charge a LOT like I do, be sure to check your batts every month. It also helps to add Minn Kotas battery conditioner once or twice a season to help battery life.
The battery manufacturer that I represent would tell you to do exactly what Jim B said earlier in this post. And yes, only USE DISTILLED WATER....I've been trying to convince Dan J to do that all week but ahh he won't listen...
It should be done when fully charged unless the plates are exposed. If the plates are exposed just fill it enough to cover them, charge it then check and fill. The reason you do it after charge is to prevent spilling the acid out as the battery is charged.