I use a Festo ball valve on the fill line and have for at least a year. There
are also three other people I know of that use the same arrangement with
no apparent problems.
Having said that, I have seen one and only one leak around a Festo coupling
on a suction line.
I believe the chance is remote. You could have a leak around a plug after usage
because the fuel line deforms. (never have seen this that I can remember)
Dee