Remember it is cycles that degrade batteries, so lowering to storage charge after every use, is in effect complete a single storage and doing this regularly will reduce the effective life of the battery maybe by as much as half of its expected life.
However I am just another uninformed airsofter in reality, and my opinion is not better than the next mans- so read what they manufacturer says- they probably know best
Airsoft batteries cost about as much as a bottle of BBs, and storage charge is not something that I worry about with any lithium based battery, whether it's phone, laptop, vacuum cleaner, or ebike. I'm not making an exception for airsoft.
They pretty much get stored as they are at the end of the day Lol. Red or green gas (whichever I've been using) but I dont top them off since I seldom run them empty , and if there are BB's in 'em they stay in 'em . In fairness I have left GBBR mags with a full BB load for extended periods and never then had an issue with the mag feeding all the BBs , although I am fussy about what BBs I put in my GBBRs