Question about Tires

After reading this thread, I took a look at my tires and saw some threads unraveling from the sidewall on the rear tire. The center tread is also pretty well worn away. They are the original tires (Bontrager Hardcase Armadillos) on a nearly 4-year old bike. The front tire looks okay. They've never flatted, but recently I did have to replace the tube on the rear wheel because the valve stem tore away from the tube (caused, I suspect, by jiggling when attaching/unattaching the pump).

I ordered a new set of tires on the cheap from Nashbar (they got pretty good reviews there).

What I'm wondering is would it be okay to keep the front tire as is, and just replace the rear? Or is it better to replace them both at the same time?

(And don't worry, I also read this thread regarding new tires.) :eek:

The front tire is the most critical tire - if it blows out, you are going down big time! So either replace both, or replace the front and move the old front tire to the rear. (As per Sheldon Brown ...gees, I miss him!)

Thank you! :)