Just the other day I tied my wife to one chair and her big toes securely to another chair and brought out a duck feather. I tickled her feet for 10 minutes with the feather alone. She laughed......yelled......wiggled.....and laughed some more.
Once the feather was wearing off (she was just smiling and not laughing) I used my fingers. She went ballistic with laughter. I ended the torture session with a vibrator with these pointy extensions. She laughed so hard, she cried.
I tickled her feet for 20 minutes....non stop.
Bottom line..........over rated and under rated tickle spots depend on the individual. There are some women who do not have ticklish feet. That is a downer. However, Someone who has feet that are so ticklish, they scream and kick when their feet are touched........that doesn't do it for me. I need to hear laughter. My wife's feet are probably a 7 out of 10.....she can handle the torture, and I get to hear tons of laughter.
One thing I found with tickling feet is that technique is really important. It took me a few tries before I learned how to tickle my wife's feet, but now that i know how to do it, it is awesome.
By the way, if feet were not ticklish in general, why would the Romans and Chinese use that part of the body as the focus for tickle torture? If it is done correctly, it works!