This is likely good advice in something like a lawnmower, but is less relevant in the Clarity because it has a specially-designed sealed (and I believe pressurized) gas tank to minimize air interaction and extend the life of the fuel.
Technically having the tank fuller still means you have less air, but I think of it this way: Let's say I use 1 gallon of gasoline every 3 months on average. If I put 7 gallons in the tank, that gas will last a year and a half, and is at serious risk of spoiling toward the end of that period. If I put two or three gallons in the tank, then I'll have burned most of it off and refilled within half a year, and there is no real risk of the fuel spoiling, plus the car is slightly lighter so I get some tiny advantage in electric range.
50 miles of electric range and another 50-150 of gas is more than enough for any unexpected driving, and I can always stop for gas if I need it. If and when I want to do a road trip, I'd probably fill up on the way out of town anyway.
Your mileage, of course, may quite literally vary. Personally I plan to buy some Sta-Bil just in case, since the EV range and 2-hour charge time means I'll almost never be using any gasoline.
This is entirely hearsay, but I have heard the Volt and Clarity's fuel system described as much more aggressive about preventing air (and humidity) from entering than most vehicles, because the designers understood gas could sit there for quite some time. This makes sense; if the car was not designed this way, people would be guaranteed to be in with service when spoiled gas screwed up their ICE, and conversely there's absolutely no reason to spend the extra money building an aggressive anti-spoilage system into a regular gas car when you can be virtually certain the only time fuel will sit in there for an extended period is if the car is in storage.
