if your fuel gauge is as inaccurate as mine, I'm sure it's safe - but find a station as soon as the "empty" warning comes on, to be safe. I managed to see at least 10 km more of range when my car said "empty".
The tank capacity is supposed to be 51 litres. Yet every time the indicator shows half or less remaining, and I fill up, it turns out I only needed to put in 20 litres at most, which is less than half the capacity. It's especially annoying if I happen to pump at a full-serve station (it's a bylaw in one part of Metro Vancouver), because the attendant will be surprised at the low amount and often attempt to top-off the tank with another squeeze or two, and I read that topping-off isn't good for this car.
Anyway, I don't see much point in trying to go all the way to empty, unless you're a Canadian on a day trip down south trying to max out the amount of American gas you can stuff in. If you're driving along and happen to see that the gas price is lower today than it was yesterday, just stop and fill up, even if it's less than a quarter tank!
I have fond memories of my last trip to Seattle in my FFH, where I purposely waited for the empty warning to come on before finally filling to the brim (saving about 40 cents per litre in the process at that period's exchange rate). That was when I was able to go 10 km extra without any issues.
My precious: 2013 Fusion Hybrid SE Ice Storm - Luxury Package (Dune), Navigation, Tech Package, Active Park Assist, Moonroof (ugh), 18" premium painted luxury wheels. Built in July 2012, bought CPO (former demonstrator) in May 2014. 6.3L/100 km lifetime fuel economy
Current temporary ride: 2003 Lexus LS430 w/navigation and parking sensors. Bought in December 2002, selling when I graduate