It's a good idea to top up the tank of your car before mothballing it (especially if you're not going to be able to run it up to temperature on a regular basis). The thinking is that it prevents opportunities for corrosion in the tank - although the high ethanol content in modern fuel causes its own problems as the fuel can split, which has a knock-on effect on various pipes and connectors. This takes about six to eight weeks to happen so consider adding an additive to your tank to prevent this.