I have done it twice lately, 340 and a 440.

Built my latest 340 in 2007 and used a cast crank that I had that was stock and KB 243 10-1 pistons and did not have it balanced. I have over 10K miles on it now and have driven it all over the country on vacations, including extended 120 MPH runs on the salt flats at Bonneville. This past summer drove it to the top of Pikes Peak for the second time in 95 degree weather and then in 115 degree weather in Vegas for 5 days.

Then back in 05 I bought a 440 short block that was freshly built with pistons that were .100" in the hole. I installed sealed power 10-1 pistons in it and did not re balance it. It runs great today. I hate to take my stuff places and try to do everything myself.

Why bore a block if it don't need it? MY 340 is standard bore with KB standard pistons in it. I honed it myself with a portable Sunnen hone when I built it.
So no it's not absolutely necessary to balance it, imo and experience.

This past summer my power brake booster started leaking causing my 340 to pop on decell, I did a compression test on it first and it had 165-170# on all 8 cyls and it don't smoke at all, none. So the the clys sealed nicely, the motor you see below. Replaced the booster and all is well.


It's idling in this picture at 13000+ altitude on the way down from Pikes Peak. It's a real test for a old car to make it to the top with a carburetor.