I keep all of mine fully charged. Yeah it might take a year off of the lifespan but li-ion tech moves fast enough that I don't mind buying amazing new cells. I do feel a little bad about wasting money like this just for convenience. But I consider it part of the price of "preparedness" insurance. If the alternative involves stockpiling 100 CR123A then I'm pretty sure I'm coming out ahead.
The test data I've seen suggests full charge storage could nearly halve the shelf-life - eg, 5 years to 70% initial capacity instead 8-10 years.
But really, the cost isn't that bad - $5-10 every 5 years for convenience, multiplied by how many batteries? How does that really compare to your other convenience decisions, like eating out when on errands or day trips, versus bringing food from home?
My strategy is mixed. I charge my li-ion batteries all the way up when they go into a light. My plan for spare batteries is to rotate them through - eg, use a battery until it's half drained or so, then charge a battery that is in storage and swap it in.
Ahead of trips or bad weather that might cause a power outage, I fully charge the batteries I want ready, then if they're not needed at that time, just use them in my regular rotation.
Also, if these are just stored batteries for emergency preparedness, look at it this way:
You either need a certain number of batteries stored fully charged to meet your preparation goal, or you need twice as many batteries half charged, but the batteries have as much as twice as long of shelf life. Long term, you get similar total cost when factoring in replacements.