As I no longer work this is from long ago, back in the 80's. I worked in a gas station / convience store and there were two girls I would tickle. One I believe thought letting me do this as a flirting thing would help her move up, it didn't. She would always sit with her shoes off, against the rules, and one foot under her sole to the back of the stool. I would walk by run my fingers up and down her foot getting a nice giggle and remind her to put her shoes on. This was like everyday she ever worked there. The other would wear short tees, also against the rules, and whenever she would reach up to grab something, often, when I was around I would run my fingers up her sides getting a nice giggle and remind her about the short tees.
I also tickle a co-worker of my wifes a lot. Her sides were very ticklish.