Yes, you can leave a lithium-ion battery fully charged, and it will hold that charge quite well. However, it will also lose total capacity. If you don't mind replacing the battery every 2-3 years, or don't mind short run-times if you choose to keep using the old cell, then don't worry about it. Otherwise, charging to 3.9v or 4.0v is a good suggestion, since that will greatly reduce the capacity loss.
I've seen
data for NMC/INR batteries (Sanyo UR18650E) and NCA (Panasonic NCR18650PD) suggesting in the ballpark of 6-7% capacity loss per year if stored at full charge and at room temperature, and maybe 3% if stored at half charge. There is a step change somewhere around 60% state of charge, so if you want to take advantage of this knowledge, the big gain comes at a resting voltage somewhere around 3.7V.
Keep in mind state of charge doesn't correlate perfectly with voltage, and this is only a test of a one model each of two chemistries. This also is based on a 10 month long test project, so it's possible the rate of capacity loss accelerates or slows over time.
Either way, I'd expect more than 2-3 years of useful life out of most cells. Hopefully 5+ years, even for cells placed into storage fully charged and ~10 years if stored half charged. The effect is more pronounced at high temperature, both on capacity and internal resistance.
Fortunately, a half-charged 18650 battery still is pretty useful. If you're not using the light often, you might not even need the full capacity anyways. If it's not a ready light that you might need to use for an extended period of time unexpectedly, meaning you'll have the chance to charge before heavy use, storing half charged is a great strategy.
It is wise, however, to check the batteries once a year to make sure they're staying above 3V, and to unscrew the tailcap of lights with electronic buttons instead of clicky switches if you're storing the batteries in the light.