This is one of those stupid arguments that there seems to be no right answer to.
FWIW...my take....(long answer)
I turn off ALL my bikes with the key....period. That way, you don't leave the ignition on accidentally and come back to a dead battery.
In most bikes...at least all of mine....the key switch is actually two switches in one. One of these two is actually wired in series with the engine cut of switch...so turning off the key , does the exact same thing (electrically) as turning off the kill switch! But the key also turns off the ignition, computer etc.
Many owners manuals say to turn off the bike with the key!
My Honda Cub has no key...it has an on / off switch (similar to a key but with no key) in addition to an engine cut off switch...the manual says to shut off the bike with the on / off switch, not the engine cut switch.
OK all that being said..the other side of the argument...
I am a ride coach for the Motorcycle Safety Foundation....
Company policy, on the range....
Motorcycle is always shut down with the following procedure...
Thumb...key....valve....
Thumb is the engine cut switch...key is the ignition...and valve is the fuel valve...
The ONLY reason we do it this way, if for inexperienced riders...their safety, the safety of the coaches, and the other riders...that the engine cut switch is right by the thumb, and it disarms the engine the fastest way. Also to train the mind hand coordination to use this cut switch in an emergency.
BTW...we are forbidden to use the term "kill switch"....."kill" is politically incorrect....LOL....really!
So short answer...do which ever way suits you!
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My dealership emphasized that I must use the kill switch before turning the ignition off. What would happen if I forgot to use the kill switch and is there a fix if it awakens gremlins?