You have to take those official recommendations with a grain of salt. There is countless studies showing that charging a Lithium battery to a lower state of charge is better for it's overall life. The charge limit slider in the car goes down to 50% for exactly that reason. It is better to charge to only 50% rather than 80%. When yor daily commute uses 20% capacity, it's better to charge to drive it from 50% to 30% daily than 90% to 70%.
Now the reason Tesla doesn't recommend that is because it would have all owner worried. They would have that feeling in the back of their head that they are harming the battery every time they charge it to 80% or 90%. And to be fair, that would be counter productive. The difference in degradation between charging daily to 100% and 90% is significant. The difference between 90% and 80% is less significant. The difference between 80% and 70% is even less and so on.
I charge my car up to whatever miles I need the next day plus 20-30% SoC. So If I need 100 miles I charge it to 150-160 miles. If I don't need it at all, I charge it to 50% (lowest setting). If I go on a long trip, I charge it to 100% just before I leave. If I'm unsure how much I need to drive the next day I usually go with 80% or 90% SoC.
My understanding is that 50% state of charge is the least wear on the battery. So, you'd put less wear going 60% to 40% each day, than 50% to 30%. However, there is a reserve held to keep the battery from bricking, so the real numbers are probably somewhere around 55% to 35% if you want to optimize for least wear.
Some years ago, Tesla shared metrics on testing done for the 18650 batteries in the S and X. The wear was roughly logarithmic to 10%. For example, charging everyday to 100% and draining to 0%, the battery might last 700 cycles. However, 90% to 10% cycles may have a lifespan of 7,000 cycles. 80% to 20% -> 70,000 cycles, and so forth.
If you charge every night, that's 365 cycles per year. So the 100% to 0% usage might get you 2 years of life on the battery. 90-10% might get you 20 years. etc. That's probably enough for most people.
My Model S is a June 2013 build. I charge to 90% nightly. I've done several 100% charges and probably about 35,000 miles of road trips around the USA using superchargers. I'm the second owner and do not know the original owner's charging patterns. My 100% is now 243 miles. The original rating was 265 miles. So, about 8.3% degradation on a 5.5 year old pack with 102,500 miles.
It's my opinion that 90% nightly charging is fine for the majority of owners.
