Charge to 100% or not?

welcome!
for a plug in prius, toyota has already taken care of that for you. what you see for charge level on the dash is only the percent charge available for use.
toyota has incorporated top and bottom end buffers, so feel free to charge away, these are very robust and long lasting batteries.
nothing you can do will lengthen or shorten the battery life or performance.

for all electric vehicles, 100% of the battery is used to get the most range for the dollar. to help counter that, most have liquid heating and cooling to preserve battery life and charge/discharge effectiveness.
when 100% charge is not needed, 80% is a good rule of thumb. and the same goes for the bottom end. plus, you don't want to be searching for a charger with only a few miles left.

 

100% dashboard is actually 85% true. This video I captured is a drive of the full EV capacity with an ODB-II app to show the difference. What Toyota shows you is just the usable portion of the battery-pack. There's a buffer at both ends for longevity, as well as some capacity saved for hybrid driving after EV depletion.

 

I would charge it to full whenever you need to inorder to stop the ICE from turning on.

When you read that a battery only lasts a certain number of charge cycles, that is different than the number of times you plug the car in. A cycle is going from 0-100%. If you charge it from 25% to 75% two times, that is one charge cycle, not two cycles. As mentioned by others, charging from 0% to 100% is actually only charging from 15% to 85%, so it is never truely being 100% charged regardless. However, keeping the battery as close to 50% as reasonably possible should prevent battery degradation. Thus charging from 25% to 85% may keep your range from dropping as much when the car is old with a lot of miles, but I don't think the difference is worth it unless it doesn't affect how much gas you use.

I have pretty extreme charging habbits and have noticed very acceptable degredation. I charge my battery from close to 0% all the way to 100% right before I leave home. Then I stop midway to work and go for a nice long walk and charge it from ~35% to 80-100% at a free public charger. I will the charge my car at work in the late afternoon from ~35% fully to ~100%. This gets me to work 35 miles away and back without any gas about 6-9 months out of the year. Thus, I estimate I have cycled my battery almost 3,000 times (~84,000 fully EV miles of my 88,500 total miles) and have only degraded it from about 6.1 KWh to 5.7 KWh of useable capacity over almost 5 years. The loss of EV range is barely noticible. In fact, I didn't notice it at all until I got new tires. I think this is about as extreme use as you can get without leaving the battery siting in 100+ degree weather fully charged all of the time for days on end.

Now that I think about it, I have pretty much maximized both number of charging sessions and the number of time the battery reaches close to 100% charged (minus the buffer). I wouldn't worry much about either. If your commute is significantly under 25 miles, just charge it to the point you can easily make it home without any gas. If it is close to 25 miles or longer, just charge it to full.