On the one hand, sometimes my girlfriend and I tickle each other and sometimes she gets the better of me. She can be relentless. I'll plead for her to stop, and I really truly want her to stop because I can barely breath, but she won't because she's interpreting my laughter as a "keep going" signal. So I've had to resort to ruthless counterattacks or ruins-the-fun physical defense (grab her wrists and force her to stop), which makes me feel like a jerk. Thus I fully support the "no means no" rule when it comes to tickling.
On the other hand, when your kid giggles and says "no..." but clearly wants to be tickled, you should tickle them! Just keep a careful eye out for signals that the fun is over. What kind of heartless monster trains kids to be as literal as possible all the time? Why not strive to teach them about appropriate context instead?
This post has made me realize that I hold these conflicting opinions simultaneously.