Ask HN: Thank you note after startup interview?

Is it common/expected to send a thank you note after an interview at a startup? I've always regarded sending (and receiving) them a waste of time because they're more fluff than anything, though I understand they're more important in traditional business environments. What's the etiquette with tech startups? And if you don't send one, does that reflect poorly on you or may lead to not getting hired? Thanks for your insights.