If the circuitry is working properly, leaving it connected to the USB cable after it completes a charge-cycle will not damage it, nor will it allow it to overcharge...
However, do you trust the circuit board?
I never charge any Li-Ion or Li-Po battery without being able to be "near" it while it charges, and never while I'm sleeping... better safe than sorry, you know? There's always that one chance in 10,000 that something will fail, go wrong, or blow-up.