I was unhappy with the 5/16 threaded rod that the Printrbot comes with and the Z-ribbing artifacts. The problem is the stepper motors have an uneven amount of steps with the 5/16 rod. When you print something with the 5/16 rod, your layer heights will be off every x number of layers unless you choose very specific layer heights. By going to 5mm threaded rod(mine cost $15 shipped from McMastercarr) you can change your steps from 2267.72 to an even 4000 steps. This gives a nice even layer height all the way through your print. Also, by adding dual nuts and a spring, any backlash is excluded. The other issue is the 5/16 rod coupled to the 5mm motor shaft is a poor design, by using 5mm threaded rod and some tubing, you get an even coupler and no z wobble.
This is hands down the best upgrade I have done on my printrbot, it improved my prints by a huge amount, if you do any upgrades, this is the one to do, best $15 bucks you will spend.