Oil, dirt on surface before mounting is what usually make them not sit properly. You can tell on surface if it was clean or not.
If it is clean, properly tightened, Not too much, not to little it will not get loose. If it does get loose, key will go poff instantly.
If you can align it without key that is just as good as key have only this purpose, nothing more!
Too much tension will not let material work as intended and it can vibrate loose just as if it has too little tension.
Example.
Sometimes you have hot cranks and sort of ignore it and assemble. As it cools it retract and wheel come loose as it then has no proper tension.