I've never had anyone straight out ask but one of my ex-gfs would like to initiate tickle fights where she would always get tickled more than she would tickle me...so I guess one can conclude that she liked getting tickled and tickling me first was her way of asking to be tickled. BTW, she was really really really ticklish.
One time in college I was at a friends house and a bunch of us were kicking back watching T.V. I heard one girl say to her boyfriend, "Can you tickle me? It takes away my stress." The guy did tickle her foot briefly and it was obvious that she was ticklish. I guess like my ex-gf she was a ticklish girl who liked to be tickled.