I’ve never heard of overfilling a battery. Even with non sealed batteries you could never overfill it.
On my mower battery that I bought this spring (one of them) I put the acid in and used it up. Took it back when it didn’t take a charge. I have no idea how but they pointed out two of the cells were over filled and two were way under filled. Sure enough I looked at it and there are lines on the battery for full and minimum. So bought more acid, removed some from the over-filled ones and filled the others and bingo it worked. So yeah it needs to cover the plates but not filled to the top anyway. Only matters where the battery is bought dry and you have to fill it. On mowers, motorcycles, snow mobiles etc.
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I recall a special bottle for topping off batteries @MikeInNH. It seems it’s important to leave enough room for the battery to vent gases produced from charging otherwise the gases will build up enough pressure to force liquid out the top and the build up of the gas can worsen the possibility of the hydrogen exploding. But could things have changed since the flat head days?
Back in high school when I worked in a gas station and topped off batteries, I often filled them too high.
If we saw the car a few hours later after they drove for a while, often the top of the battery had acid all over it from the cells. Overfilling a battery that’s low on charge will cause acid overflow during charging.
The gas station owner kept telling me to stop doing that, and eventually I got it.
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In the old days we use to top off batteries with distilled water. There was never a warning about over filling. It was damn near impossible to tell if you ever did over fill it.
Never had an overfill Problem. You stop when a meniscus forms at the grove. 
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