I've updated firmware on wrt54g routers appoximately 50 times without permanently breaking a router.
That said, any firmware upgrade has associated risks. If you lose connection in the middle of the transfer, you'll end up with a corrupted firmware image, which means your router will not boot. A router that doesn't boot is affectionately known as a 'bricked' router.
Wirds.net is not responsible if you brick your router.
If you do run into problems, there's a trick to get new firmware on your router. When the router boots, it looks for tftp server that is sending firmware. How long it waits depends on if your boot_wait variable is on or not. If it is on, you'll probably be successful on your first try. If not, it will probably take a few tries to pull it off. I've fixed many unresponsive routers using this tftp method found at openwrt tftp guide