Years ago when I had a Subaru Outback, I had to park it for about 3 montsh and was worried about fuel 'souring'. I added a fuel stabilizer as suggested by some. It's an additive that is supposed to prevent souring. On returning from the trip, the check engine light came on and the car went into limp home mode. It cleared itself after siphoning the fuel out and putting in new fuel without the additive. Twice after that did I have to park the car for about 3 months, and I did not use any additives at that time, and had no problems.
In the old days it was recommended to top off the tank before long term storage, to reduce condensation. With the Prius fuel bladder that is not needed, and I would think with the Prius II it would be better to park with very little fuel, so right after resuming operations you can top off with fresh gas.