It kind of depends regarding flying without a fully charged battery.
The crux is that when the batteries are left alone for a few days, they start to discharge themselves automatically to a certain level. So there's a risk when you grab a battery that's 90% full and hasn't been used for a while that it's discharging. When you takeoff in this state, the battery level may suddenly drop rapidly, and before the quad is back, it is depleted.
If I recall correctly, all low battery flights that went wrong seemed to be caused by this. Though, anyone please correct me if I'm wrong.
However, if you've made a flight the same day or one before and the battery was around 60 or 70% when you packed it, you can actually still takeoff. I've done this countless of times, and I'm sure many others have taken off and landed multiple times with one battery. Just don't go far or do any risky flights, because obviously, the less power is left, the shorter the flight time is. And 60% of 17 minutes effective flight time, isn't a whole lot. Flying without a fully charged battery certainly has its risks, so always fly more careful when you do.
But that said, in general, just use fully charged batteries whenever you can.