Okay, electricity takes the path of least resistance. So theoretically, you can easily be fine welding on your truck without disconnecting the battery (as so many of you have stated). As long as the ground clamp is next to your welds (and thus the current flowing through the metal to your ground), you'll be fine.
As long as the ground clamp isn't across from the battery or something funky you can be fine without removing it.
however, not everything goes as theorized,so its cheap insurance/safety factor to just disconnect it.