Don't know if this is specific to the Droid X. My Nexus One doesn't need rebooting ever. The last time I can remember rebooting it was when I forgot to charge it overnight and it died.
Maybe it's a MotoBlur specific or Droid X specific problem? If I recall correctly, my Nexus One rebooted when it got the 2.1 update1 and when it got the 2.2 update. Other than that, I think the only times it's ever rebooted is, like you, when I've forgotten to charge it and let the battery die. :-)