I've seen welding and mods around here (and other sites) that are far more "dangerous".... and I've taken plenty of factory tanks out with lots of evidence of gas leakage around the fill connection and other ports. Gas fumes are heavier than air... the tanks exterior is "open to atmosphere" and therefore the fumes "should" fall away and not be in a confined space.

Scares me more to fill a plastic gas can where I can SEE the fumes rolling out the top and down the sides - and I'm standing right there with my face and hands close to this! Something as simple as static electricity could set it off. WTF

I don't think this is a big safety issue - and it's not at all if done correctly.