For most tickling, I prefer a shirt on the lee. I want to minimize the chance that I'll scratch them. And the shirt gives a little extra protective barrier. Not too thick though. 🙂 Because actually a thin shirt can sort of do for the upper body what nylons and dress socks do for feet. Not exactly the same, but similar. I'm a switch, so as a lee, a shirt on me definitely doesn't reduce the effectiveness for the ler. And I like being tickled with long nails, so like I said about the shirt providing some protection, it allows them to use their nails without fear of feeling like they're hurting me. Soft, light tickling with fingertips, nails, or a feather is better on bare skin though.
Going under the shirt is fun! It feels sort of like an invasion. Like a barrier has been breached. I love it when it's done to me. I have yet to do that to someone I'm tickling, though it's on the list. I know when watching someone tickling someone by going under their shirt, there's just something about seeing their hands and fingers obviously moving under there, and imagining how what they're doing feels for the lee.
But as always, nothing is hard and fast. Always open to experimentation, especially if there's something who I'm with wants to try.