I've only really shared my fetish with platonic friends twice, in two very different situations, and with very different results.
The first was my best friend. We click really well together as friends, and are very comfortable with one another, capable of talking about pretty much anything. Our conversations tend to meander between topics, and we just kind of say whatever we feel like saying. At some point or other, a conversation switched over to sex-type things, and we were talking about our preferences and what have you, and my fetish just kind of came out in the most casual way, like: "I dunno, I kinda like redheads. And freckles, freckles are cute... and she'd have to be ticklish. I've got kind of a thing for tickling, to be honest, heh." He paused and asked about that, I elaborated a little, and he shrugged and told me about an unusual kink of his own, and te conversation moved on. It's been mentioned only once before, when discussing puns and teasing one another; he complained jokingly that he could only think of one pun with which to make fun of me, that being something to the tune of "tickling my fancy," while I could come up with myriad puns to make fun of him, if I wanted to. And I did. It was all good natured, don't worry... XD
The second time was... eh, less positive. My sibling's then-best friend, with whom I was also on friendly terms, for some reason got it into her head to ask if I was ticklish. Probably just being playful, not even aware of what that question does to our kind of folk... Being a trap question, of course, I knew she was going to attack, and I didn't particularly want to be tickled by this girl--she was younger than me, she was my sibling's best friend... It would have made me feel awkward and uncomfortable if she tickled me. So, in a blind panic as she was advancing on me, I just kind of blurted it out. It worked, sure, stopping her in her tracks, but... eh, she still gives me weird looks about it to this day... >w>