Just remember when you "Upload via Programmer" sketches, it will overwrite the bootloader and you will not have Upload capability from a USB2UART device (i.e. FTDI, duemilanove, etc). Only the USBasp or SPI device will work. You will have to reburn the bootloader to put it back. Also, after you burn the bootloader with SLOW_CLK on, the fuses will be set to run the USBasp at full speed with SLOW_CLK off.