Recent smartphones learn your patterns and don't fill up until the morning. For instance my phone currently is stuck at 80% and says it'll be fully charged at 7AM
I really wish this worked for times other than the mornings. I work in the late evenings most of the time, so thus its pretty normal for me to wake up at 9PM (like today) - however, the only time it ever seems to do the trickle charge is for when I set an alarm for say 10AM and plug my phone in at 3AM.
It's not about the phones learning the patterns, which as you've said recent phones have corrected that, but due to energy sustainability and trickle charging (once it's fully charged, it will keep energy flowing in order to keep itself fully charged, or to keep itself at 85% as other comments have indicated) - you should unplug once it's fully charged in order to conserve electricity. A phone may not seem like much, but every bit counts and it permeates throughout the rest of your personality/devices and adds up, especially when everyone does it.
I charge my phone when it needs it and unplug it when it's done. Sometimes that lasts two days, sometimes it's half a day, sometimes I plug it in first thing in the morning, sometimes I plug it in while I'm driving home at night.