I have seen an argument for squeezing as much in as possible at the end of a ride, especially in the colder months, if the bike is not going to be used every day to leave as little exposed metal inside the tank for condensation to form on and contaminate the fuel. I have always taken this to heart so, like Mr a Nick above I have always filled my Bandit up to the bottom of the filler neck.
That bike had a centre stand so made it easier but did sit astride it when my poorly back meant it was difficult to use the main stand a few months ago. I've never had a problem with staff not letting me fill up on the bike or with my helmet, flip front, on but have heard from many who have.
My new Glee also has a centre stand so will probably do the same with this bike. I hadn't thought about fuel spillage causing problems in the nether regions whilst fueling sitting on the bike but worth bearing in mind as had this several times with leaking tanks on my MXers years ago and quite painful!