In most cases, when you over tighten the nut on the mandrel or over tighten the tailstock you put so much pressure that you end up bending the mandrel rod. The result? You end up with oval barrels rather than round barrels.
The solution - simply remove the live center tail stock and replace it with a Pen Mandrel Saver. With this niffty contraption, the live center pushes against the bushings rather than applying pressure on the rod. Brilliant.
I got some made for Timberbits in Australia. Check 'em out - http://www.timberbits.com/pen-mandrel-saver.html
Here's a quick video demonstration.