I've just bought a 2019 38 kWh Premium EV. Tha manual recommends charging to 100% once or more per month. Reason for doing this is to balance the (many) cells making up the HV battery. What you do NOT want to do to a Li-ion cell is over-charge it. So as the charging approaches 100%, there are a variety of techniques used to balance them, but it amounts to getting them to all sit at the same final voltage after the charge. I've noticed my charging to 100% gets very slow at the end, it's down to about 2kW around 98-99% - and I suspect this is a side-effect of the BMS (Battery Management System) doing the careful balancing & fine-tuning needed to keep them in tip-top shape.
The first time I charged to 100% was for a long trip the next morning. I actually charged to 80% on a Rapid, then early the next morning got up early, went to the Rapid planning to charge to 100% in (I thought) 45 minutes, go home, pack for an hour, and go. It actually took 1hr 40 mins to do that last 20% charge! Luckily I had plenty of time, and no-one else turned up wanting the Rapid!
It's reckoned to be a good idea to keep the battery between 20% & 80% charged for the longest life; Amperas have been doing this for years, they only let you use 65% of the true capacity, and hardly any of those 2012-vintage batteries have failed or had any degradation that the owners can see. One of them has done over 400,000 miles, still on original battery, so 200,000 of those are purely electric, the rest petrol-though-exercising-the-HV-battery mode. GM ended up with shelf-loads of replacement batteries they'd expected to have to supply under warranty, and had to flog them off cheap as there weren't needed!
Li-ion batteries don't like sitting at 0% SOC, or 100% SOC, for long periods of time. The EV mfrs leave some space (buffer) at the top & bottom end of the HV batteries, but it's not huge like the Amperas is, maybe 3% at each end, something like that. Otoh the newer chemistries used now should be making the batteries even more reliable than Ampera ones are, so maybe it's safe to leave Ioniqs at 100% SOC overnight. Trouble is, we won't know for a few more years yet if this is the case, so I for one prefer to sit in 20-80% range normally, and only go outside that region for as little time as possible. I wouldn't leave my EV at 100% for days. A few hours I don't mind. Now we're in lockdown, I did my last 100% fillup at home to balance the cells, then ran the heater with the windows open for a couple of hours to drop it to 80%. And that's where it's been the last 2 weeks.