No, once you're unlocked, you're unlocked until you want to flash a stock ROM and lock the device to bring it in for Service or Repair. Otherwise, once you're unlocked, you're good to continue flashing to your hearts content.

You should def take some time and read through these threads here:
http://forum.ppcgeeks.com/showthread.php?t=69854
and
http://forum.ppcgeeks.com/showthread.php?t=61333

You only lock the device if you're bringing it in to Sprint for service, replacement, or repair. Flashing Custom Roms is a no-no, so we don't want them to figure out what we did.