Both my husband and I wear knee high compression socks on a regular basis and by doctor's order. We get the medium compression ones, 15-20 mmHG. Anything lower in compression is too loose for us and anything higher is too tight. Both of us have generously sized ankles and calves. Be sure to check with your doc, but especially if you haven't worn them before, the 20-30 ones will probably be too tight.
My favorites are Sockwell Circulators (on sale now for $20 per pair at Travelsmith). I have other Sockwells and other brands including Travelsox, but none work as well for me as the Circulators. They are made of Merino wool and rayon, and they wick away moisture and keep my feet warm and dry, but not hot, even down here in the desert Southwest. I have never had a blister anywhere on my feet when wearing them. FootSmart is another place to find then and many other brands and styles of compression socks.
Some brands are tighter at the ankle and more generous at the calf but when I take them off, my legs look like a tied off balloon animal. I am glad to take the Sockwells off, too, but I find them comfortable and comforting to wear, and I'm sure they helped keep me from spraining my ankle when I stepped off a curb sideways about a year ago.
I wore them all the time with boots (or sometimes with shoes or even sandals) on my last 3 trips to Europe. I wore them today at the gym for my workout. Most people don't realize that they were first invented to improve circulation and performance for athletes.
As for the plantar fasciitis, I had a horrible time with it, but don't anymore. I was given exercises to do and told to get different shoes. Up until then, I was a Birkenstock and Haflinger person for many, many years. I learned that their footbeds were just too hard. I switched to other brands of shoes with softer footbeds but good arch and other support for my feet. That change in combination with the typical exercises have kept me pain free for the last 6 years.
As many say on the forum YMMV and what works for me may not work for you.