If you unlock the card after taking out of your cam and before putting it in the slot, you will save 1 lock-unlock
cycle, which should give the tiny plastic thingy a longer life ...
Or use (in cam with CHDK) Miscellaneous Stuff -> File Browser to delete files. see page 50 in the user guide
(link in my signature) -- then you can leave the lock on.
The reason ? The lock on an SD card is not 'hard', it's up to the controlling software to check if it's on, so CHDK
just prevents the reader in the cam from checking it.
hope that helps,