Depending on whether you are using a powered usb hub or just the usb jack from your computer, the rate of charge may vary. Generally, a usb charge is slower, and is known as a slow trickle charge. That is actually better for your battery than a faster charge. Fast charges can heat your battery and literally reduce your battery's lifespan, or ability to hold a charge, all for the sake of convenience.
It should be as safe to charge your device from a USB source as how reliable your computer regulates it's voltage, which is 5 volts. So, that really means it should be safe.