It depends. What is it they are using? For me, if they are using their nails or something hard or sharp, it would be best if I had socks on. Personally, I have very sensitive skins and women who use long figernails to tickle with feel like razor blades scraping against the skin (unless that just happened to be from someone who is not experienced in using them). It's been my experience that when my socks have been slowly removed and a female has caressed my foot, stating how soft it feels, etc., then is when I start getting aroused and the tickling just adds to it.
For me, the softer it is, the more ticklish it is to me. And to tell you the truth, I really don't know why. I've ready plenty of stories, alot of them speak of women using their nails, which never did a thing for me. I am not sure if it's because the person who first started this when I was a teenager had short nails and had very soft fingertips, was also very gentle to me as compared to some of the stories I have read online, as well as the things she said. For example, instead of saying "c'mon, you can take more, you wimp!", she would say, depending on the position and situation, "c'mon, why don't you admit it? You know you love it! I love it too!" "You're never going to last! Don't laugh! Oooh, you're smiling, the pressure is on! Whatever you do, don't laugh. Your face might crack." "Look at him, he's about to break!" Oh, was she ever good (at least breaking me anyways)! What always got me was her silliness. Like when she grabbed my foot, took her forefinger, slightly touching each of my toes one at a time while making some silly sound made me laugh and at that moment she would accuse me of being ticklish, even if it didn't exactly tickle, and she would start the real tickling on my poor defenseless foot.
If any of you ladies out there are reading this, where exactly did you learn this skill? Did you have brothers who did this to you? Did you learn it from your other friends like at sleepovers or slumber parties? Snuck downstairs while your parents were asleep at night and pop in one of their x-rated video tapes? Whatever it was, I am sure you don't regret it!