There's so many variables to this, I don't even know where to start. Keep in mind most models, who would be deathly ticklish in an everyday situation, psychologically prepare themselves beforehand. Plus, I suspect models who are deathly ticklish don't tend to apply for tickling shoots. lol. I have a lot of friends who are ticklish like what you describe and they would NEVER do a tickling video. Also, they're not models. They're everyday girls who just don't get into that kind of stuff, so there's not as desensitized to it.
What I actually get PO'd about from time to time is the ticklers skills. I've seen SOOO many producers just lightly wave their hands over the models ribs instead of digging into them. Almost rarely do they get their nails under the toes and don't squeeze the holy bejeezus out of their inner thighs. The upper inner thighs are an extremely neglected area with lers.
There's also a psychological factor to tickling not often accounted for out of fear it might offend people (although I don't mean it in an insulting way like some people do). A lot of people I've met who aren't ticklish, or who are only moderately ticklish, have trust and comfort issues. Even then, it depends on who they're with, the situation they're in, etc.. If they're not comfortable around you, they're not going to be ticklish and they're going to be "on guard." So, for those producers/lers who think models are either ticklish or they're not, you need to consider there's a whole background that happens before the tickling video which accounts for some of the "tickle resistance." Even knowing you're going to a model for a tickle video for money can be a factor in how ticklish a person is. I mean, it could even be what mood they're in that day that accounts for sub par shoots.
Oh, and of course Rhiannon's comment was ridiculous. She obviously doesn't understand the psychological factor involved. Tickling is a "holistic" experience and not just some physiological reaction.