The battery cell balancing which is important, only occurs after the full charge state is met and the car is still plugged in. As per your owners manual, you should leave the car plugged in whenever possible once certain temperature thresholds are met. Bottom line, charge nightly and always leave it plugged in when at home.
Re: which is better, I don't think there is any scientific searched answer to that. I typically charge when I get home (slightly higher than 1/2 battery) takes 2 hours at 240V but usually leave it plugged in for more three hours etc, (cell balancing?) then unplug it. I leave it plugged in during winter when it is near freezing so it is ready for preconditioning. No temperature extremes here. Rarely below freezing, rarely above 80F. As far as I know, the battery is still at 10.3 Kwh from the day I bought it over three years ago. I've not discharged to start gas since COVID (March 2020). The range was always at 60 Km (preCOVID), this summer it is at 62 Km. Not that My battery has gotten better with age but because I don't do the once a month out of the valley trip and mostly the roads here are 36 mph. except for the TCH. All anecdotal but I think that's all you've got. The manual says plug it in all the time (nothing wrong with that) but I think that is mostly to cover their ass for the extremes, cold weather, hot weather, cell balancing etc. and aiming for the lowest common denominator of customer ownership like they can't figure anything out for themselves. Sorry for the mix of measurements, I am bilingual speaking English and US measurements and metric as well as linguistically, speaking English, American as well as some Australian. 😝
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