Purely a matter of style. Some end tsuki with the jo at the level of the tanden, with the hands lined up the way they'd be if you had been doing it w/ a bokken. From what I remember of another style, it ends with a spiral, the butt of the jo pretty much at/by your armpit.
When in Rome....
I learned it the first way you mention, but now I am at a school that does it the second way. They are both good, but I find that the second way gives you more of an extension, and is in keeping with the way you would use a spear. Again, it is just a matter of style and preference. It's all good!