Nathan
Do take all precautions possible when working on or around them. The fuel may be gone but vapors WILL remain, bad things can happen fast, I've seen and heard the results.
Ed
Good lookin' out Ed. I've welded on gas tanks before. I do a full flush once with water, a full flush again with dish soap, and then another flush with water. Open flame test shows no flammability. After welding, I flush with methanol to absorb any residual water before filling with gas.