Breaker lock-outs are certainly useful; I have several types. There is no one type that works on all breakers, and some breakers will not accept any of the devices I've seen.
In addition, there are devices for locking switches, plugging fuse holders, covering plugs, etc. I'd stay away from the kits, and buy them as you use them.
I'd also stay away from padlocks; a zip-tie and a tag seem perfectly adequate to me- and eliminates creating another hazard by leaving the door ajar!
