Well, let's see:
Every electrical switch (lights, etc)
Every electrical outlet (though these won't always arc, the switches will normally arc)
Every electrical appliance
Your furnace
Your hat water heater
Your gas fireplace
Candles
Cigarette lighters
hmm, I'm running out of ideas off the top of my head. But I think you get the point.
Although the risk of ignition is moderate to low at any given time (gasoline needs to be between about 14-20% mix in the ambient air for it to ignite, which is fairly substantial but you do need to consider that the vapours are heavier than air and will collect over time) it's a risk a wise person wouldn't take.
In the garage, any fumes tend to be ventilated out the openings before you are likely to get a dangerous buildup. Also, there are somewhat fewer sources of ignition. Also, you don't sleep in your garage. For those reasons its a relatively low risk. In the place that you sleep? Not so much.
Simple precaution is to drain the fuel. Not a big deal.