[JLMounce]

Modern shafts are definitely very finely balanced. On my 2007 Mustang GT/CS, I encountered a vibration after swapping to a 4.10 ring and pinion. On those cars the drivetrain is balanced as a complete unit from the transmission to the rear-end.

In an effort to get rid of the vibration after the gear change I clocked the driveshaft 180 degrees from it's original position (which helped slightly). There was a sticker on the driveshaft stating if it was dropped, it should be replaced. Not inspected for damage, not repair if damaged, replaced.

When you've got a big heavy tube rotating at several thousand RPM, small discrepancies in weight start to add up. It may not be a reliability or safety detriment to the shaft itself, but it could cause a significant driver annoyance.

EDIT: I realize here that if you're wanting to paint or powdercoat the shaft, you might be doing so because it's going in a higher end show car, one which may not be driven all the time. If the car is a concourse car that gets driven seldom, and likely never at speeds over a moderate cruise, it probably doesn't matter. It's the car that will be driven often at city and highway speeds that this becomes more of a concern.