I kind of think our community should have some kind of rating system on feelings about being tickled -- assuming the person is ticklish to begin with:
1.) Loves being tickled in most any reasonable scenario
2.) Likes being tickled, provided they don't feel it's too "extreme," such as it doesn't last too long, they're not restrained, or it's done in a particular way
3.) Doesn't exactly like or dislike being tickled per se, and will be okay with it provided some fairly significant conditions, like their partner has to be the one doing it, or payments are involved - although if these significant conditions are met, she'll be okay with it
4.) Definitely doesn't like being tickled, but won't react too badly if it stops after a few seconds -- but will react adversely if it continues for very long, no matter the scenario
5.) Absolutely despises being tickled, and will react extremely negatively to it on any occasion
I realize there's some people cross-over between aspects of these, and maybe someone can improve on it, but this would be my basic outline.