[mao511]

Once you unlock you can flash away all you want. If something happens to your phone and you need to take it to sprint then you lock it, and you would have to install the stock rom also. I keep those 2 files on my laptop just in case. And it's also a good idea to flash back to stock and clearing storage after you flashed a couple of custom roms. It's a way of cleansing the phone so the new rom can function up to its full potential.