Tyler, Just think of the bar blades as a weighted flywheel. Even your vehicle wheels are basically a flywheel or rotating mass that must be in balance. It is the same for aircraft props. Flywheel themselves are not always completely round either. A good example is the dog bone flywheel that Stihl uses on their backpack blowers.
Now there are decks on some equipment that both blades attacking the grass at same time. These have longer blades that would be constantly hitting each other if not timed by a timing system. On these systems the blades are timed 90 degrees of each other but they still need to be in balance. Some of the Cub Cadet walk behind commercial mowers uses this system and there is also some John Deere riders too. I have worked on both of these OEMs but don't know any models numbers off the top my head. These are usually two blade systems. So far I haven't seen a three blade timed system.
It is however much more common to have a v-belt driven deck with one side trailing the other which simplifies the deck setup.
Over the years so far I have only seen a couple after market blades that were too long by few thousandths and they ping every so often due bearing play. The solution was to shorten the blades a little bit while keeping them in balance.
