If you have a lathe with a flat-topped headstock as I do, you know that it's impossible to resist the impulse to keep oft-used tools there. After all, why did the manufacturer make it flat?
This practice has a real safety issue. If a tool is accidentally brushed from the headstock tray it can fall into the spinning chuck and be propelled in some random direction. Many of those random directions intersect your face, sternum and other important parts of your anatomy.
Get some type of box with walls and secure it to the headstock so that it's impossible to sweep it or the tools it contains off accidentally. Here's my approach...
Note that this style of tray provides convenient detents in the walls to hold the chuck wrench. I only discovered that after mounting the box.
Many of us, myself included, are guilty of engaging the backgear to lock the spindle in order to remove the chuck. It's not too bad if you religiously clean and oil the spindle threads before mounting the chuck but I still suggest you abandon the practice and make the spindle crank I described elsewhere.
Nevertheless, people will do it. If you do, it's just tooo easy for your muscle memory to take over at some point and make you reach for the start switch. Broken back gears are expensive and difficult to replace. Make a lockout to remind your hand when it involuntarily reaches for that switch.
Every lathe is different but here's what my switch looks like...
and here the wooden lockout that prevents any motion of the switch.
One other safety remark for folks who own smaller or older lathes that don't have built-in drop-out power switches. If the power drops out your lathe/mill/grinder/etc. will resume running when it comes back on unless you remember to switch it off as soon as the power fades. Try to get in the habit of making that the first thing you do, even before reaching for a flashlight, when the power drops.