I have heard that you should keep your fuel tank as full as possible,
to reduce the exposed surface on the interior of the tank, on which
condensation (ie. water) can form.

Is there any truth to this?

Also, is it ever dangerous (other than the risk of spilling petrol
in the process!) to fill up the tank completely, to just before where
the cap goes? I'm referring to the tanks where this is physically
possible, of course. In other words, is there ever any need for
'a bit of air in the system' ?