Good question! I am a new LV speedy owner and I was wondering the same thing. I have been leaving the lock on all of the time. I think it adds to the charm.
The lock and key comes with the Speedy when you buy it or at least the ones I have did. Sometimes I keep the lock & key in a pouch inside my bag, or sometimes they're on the bag or in a drawer. lol, depends on my mood.
i don't leave my lock on. after a while, it tends to bend the flap and deform and discolor the hole. the only change over time that i want on my speedies is the patina...
I only put the lock on when I use it... kinda like a charm.
and I take it off and keep it in the pouch after I use the bag...
I try to minimize the staining of the bronze on the leather.
For my mono 30, I leave the lock locked. I bought a Hazelnut vernis cles and clipped it on the outside of my bag (for decoration too). That's where my keys are!
i leave it on, and leave the keys inside the pocket. it adds a lil something special to the bag i think. although its not on at the moment coz i wanted to do a trial with it off for a while hehe