Never heard of filling the petrol tank up! Just make sure you have additives so the petrol does not seperate.

Things I would recommend is listed below although I'm sure people have other tricks. This is only for short term storage ie over winter

1) Petrol additives
2) Leave the hand brake off
3) Over pressure the tyres to stop any flat spots
4) Ideally use a carcoon
5) Start every six to eight weeks