Balance while charging is best.

Consider this...

You have 3 glasses of water sitting side by side connected together by a hollow tube.
You close off the tubes and fill the glasses with water, however, you can't get each glass filled with exactly the same amount of water.
Then you open the tubes to let the level of water equalize in each of the three glasses. The less full glasses takes some water from the more full glasses leaving all 3 glasses equal in quantity but not-quite-full.

Now...
Take those same 3 glasses and leave the tube open and fill the glasses with water.
As the glasses fill the water flows back and forth between the glasses keeping the level of water equal until all 3 glasses are full.
Now you have 3 full glasses of water.

Same while charging a battery.