Ok, so I was triggered by this thread to do some research. This graph shows why it is unwise to go below say 20% charge, because your battery will loose power rapidly when you keep using it after that point, and the chance that you damage the battery increases if you go too low.
This article gives some insight in how to prolong your battery life. The lower you go before charging again, the less cycles you get, but - obviously - the longer your watch performs per discharge. So suppose you charge to 100% and then use the watch until there's 20% left (that would take say 4-5 days on my GW46mm BT) that should give you according to this article around 900 cycles, or - with 4 days per discharge - at least 3600 days before you need to replace the battery. That's almost 10 years. Nothing to worry about, I'd say.
If you charge your battery every day, and let's assume there's still 80% left when you recharge it, that would give you 9000 cycles or 9000 days. That is indeed much longer, over 25 years before battery replacement. But assuming you'll replace your watch every 4 years or sooner, you can do what you want, as long as you don't go too low.
If you want to store your watch, make sure it has around 50% charge left. 100% charge in storage can damage your battery apparently and too low a charge can damage your battery as well because of the ongoing self-discharge of batteries (1-2% a month for Li-ion).
Li-ion batteries can be damaged when overcharged, but a good Li-ion dedicated charger will stop charging when the battery is full. So charging over night should not be a problem, although I guess you could consider your watch being "in storage" over night at 100%. In that case, you may want to either start wearing it immediately once full or prevent it from reaching 100%, e.g. by charging it daily while taking a shower only. The article mentions an ideal charge range of 30-80% for optimal battery life. But again, even if you loose some battery life while charging over night, I strongly doubt you will notice it, assuming you will have replaced your watch after 4 years anyway.
I charged my S2 over night for 3 years and the battery was still going strong when I sold the S2 to someone else.