There were three tunings popular in vintage music I've found in my collection of songsheets - generally a higher tone was used because it has better projection and volume on stage or in a large room.
You always have two choices when playing this music regardless of tuning: to play the chord shapes as shown by the diagram, or to play the chord as shown by the key. I tend to play the shape and I generally ignore the tuning, too. I find it only matters when trying to figure something out from an old recording.
You aren't missing anything aside from a higher note with D6. Personally, I prefer lower tuning because of my vocal range and for all my low-G ukes.
