I have the time set to right before I leave. That way it's not sitting at a high state of charge for long. I have a 2013 US model so I also have the 80% option, which I normally use. If you have a 2015 US model you won't have that option. But 2015's from other countries do have that option.
I also do it to help increase battery temperature in the winter, right before I drive. It doesn't seem to do much, but I think it helps a little.
That battery charger timer can also allow you to charge when electric costs are low. In some places the cost of electricity is lower during the night. Do check with your electric company and make sure your charging stops before the price increases. I think that would be the most important.
If you can't charge to only 80% could you charge every other night? For an example, one day you go from 100% to 60% or 65%, then the next day you go down to 20% to 30%. If you'd get below 20% it wouldn't be worth it. And you may want the 100% for something unforeseen. But it would be a bit better for the battery.
2013 SL, 40,000 miles, 12 bars.