Recently while making something relatively large, I thought that there should be an option to paint all 5 sides of a block at once. This would be a temporary button a little left of the top-right corner that would be switchable.
Seriously though, we need this. After thinking of the time it took to make and paint, I realized it took me 45 minutes to an hour to paint all the extra sides, it really annoyed me to be honest.
Overall, please add this, I despise wasting time painting every single side.