Programming a 3.3V Atmega chip using USB
If you are planning to use USB as the power source, of course you should use 3.3 v regulator like AMS1117 to convert 5 v to 3.3v. If you are asking about the USB communication between computer and atmega, level converter is not required since the usb signal lines D+ and D- is 3.3 v differential signal. That is the USB standard, it is same for every usb device. More about USb http://www.beyondlogic.org/usbnutshell/usb2.shtml