Don't all in line six motors fire 120 degrees apart from the previous piston and cylinder, two pistons at top dead center at the same time correct? Is there any 180 degree in line six motors?
That being said why wouldn't you want to balance the rods, pistons and crankshaft like any other motor?
I know most of the factory specs on balancing production motors is no where near as good as any decent Hi Po balance job would be, + or - one gram on all of the rotating parts
If it is apart and you want to twist it tail to make it howl, balance it