I'm not actually sure why they have people mark the driveshaft and yokes before removal. The driveshaft is balanced separately from the yokes... I'm not even sure if the yokes are balanced at all. I've never bothered marking the driveshaft and yokes and it's never been an issue.
Now - if you were going to separate the sliding splined yoke from the end of the front driveshaft to replace the seal or something, you definitely would want to mark the two sides so you could assemble them properly again because those ARE balanced as a set, AFAIK.