I always suggest fully charged.
In most cases they'll slowly drain but a dead battery is always a bad idea.
Anything that has removable batteries should be removed.
In the case of internal batteries, just charge then store.
+1
Due to my R/C hobby, I learned that it's also good to store LiPo packs in the freezer, for long term storage. They should also be charged back to 3.7-3.8V about every two to three months.
Thank you @Wingsfan0310 and @Neunerball
Not the first mistaken advise I've ever given, I'll probably do something similar one or twice more at some point.
haha
But seriously, I was unaware and basing my answer on the results I had gotten from other types of batteries.
I've never stored an 18650, I use all 60+ of mine in rotation.
*That SHOULD have been my clue NOT to advise on something I didn't positively know the answer to.*