Personally, I've never filled air, not because I don't really know how but rather because I don't wish to without someone actually teaching me. Its the same with filling co2, I know the process, I know how fill, I know the bottle weight empty and maximum when filled. I even know how to diagnose many problems with the bottle, but without someone actually sitting me down and demonstrating it to me I personally don't think I should be aloud to fill.
...If you can't be bothered to read all that text above. I think training should be mandatory even if people believe they know how.
My newbie 2 cents (pence)
Dan