Sex is a matter of mind, not just body.
Power and domination are important elements in some [most?] relationships - so tickling someone might be a softer form of self-assertion in a couple.
In SM relationships the bottom, that is the victim, sometimes gets a strong emotion from being made focal point of someone's attentions.
The bottom is under [mild] torture, but esperiences pleasure from losing control and/or from indirectly controlling the top - the torturer.
For this reason working SM relationships require trust and understanding. As a top you have a responsibility, as a bottom you are "the only actor on stage".
Many psychologists around say - on papers and such - tickling is a form of physical domination, therefore it is bad. They mistake a fetish for harassment, and they suggest counselling.
Of course I think they are wrong. At least on a general basis - as it usually is from the precious few info they get from readers' mail. 🙂
Now, back to your question: is it bad to tickle somebody? Maybe yes, maybe no. Is it bad to pinch someone who does NOT want to be pinched? Or caressed? Or kissed? Or whatever?
As long as partners love and respect each other... it's a private matter between consenting adults. 🙂
As they say, if it makes your boat float, and lover goes along, it is [probably] fair game.
I'm sorry for being verbose. Hope I helped. 😀
Reg's!