Like the other answer, I couldn't find a specific, good study to go off of for any sort of real statement. The ones I have seen tend to be pretty low sample sizes and somewhat limited scope. What I did find that supported some experimental data directed at the type of lipos that get used for quadcopters was this: https://www.propwashed.com/lipo-storage-voltage/.
The TL;DR from that is the study did not find any real difference between a lipo stored at any normal voltage level.
With that said, I have seen another study someone did with a slightly larger sample size (5-10 batteries over several months), but I can't seem to dig it up right now. The conclusion that the study had (from my recollection) was that storage at full (4.2v) or empty (3.5v) had an impact on the battery similar to single-digit charge cycles (fully charging, then fully discharging the battery) per week. Take this with a grain of salt, but from my memory, a fully charged battery stored for a week was roughly equivalent to 2-3 additional charge cycles, while a fully discharged battery was roughly equivalent to 5-6 charge cycles.
My personal approach is to not worry about it too much. The only battery I've seen significant damage to from storage was left at empty for almost a year and had discharged well below 3v/cell. Typically I charge my batteries to fully charged if I'm planning a flight session in the next couple days, and storage charge them otherwise. This is less than ideal if I needed absolute performance out of them, but I'm not a pro racer who needs that, and essentially all the batteries I've retired have been due to crash damage, not degradation over time.
To that end, I'd recommend whatever best fits your flight style. Most data I've seen shows a lipo will generally last 300-400 charge cycles, so cutting that by 25% due to keeping them at full charge all the time seems like a reasonable trade-off if it means you're more easily able to get out and fly when you want to. If you're in a situation where you can easily charge from storage->full prior to ever flight session, keep them at storage and enjoy the extra lifespan.