I don't think the question is "Would they be offended if I didn't say thanks?". Because especially when they don't expect to hear a "Thank you", it's worth it. Or maybe they don't care or are irritated, but that shouldn't stop anyone to say thanks.
A friend of my mum's once said I shouldn't "help" the waiter if they come to collect the dirty dishes. You know, not hand the dishes to them, etc. She didn't say it because I shouldn't confuse the waiter, but because "that's not what you're supposed to do". I just thought, "This is the most arrogant thing ever" and left it. I actually got really pissed off because I was trying to be nice, "but no one asked for it". Whatever. ^^ The worst part of it was that she made it sound like, "Ugh, you're so naive, do you really think you should do something like that? That's not how things are DONE".
I just can't be impolite. Or rather... I have to show people my gratitude because, after all, they're providing a service and if they weren't there to do it, I'd have to do it myself. This world is difficult and cold enough already, so I'd rather say "thank you" than nothing.
I think free samples wouldn't be that extreme, but I still would say thank you, just for the sake of it. ^^
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"It's how they see things. It's a way of bringing class to an environment, and I say that pejoratively because, obviously, good music is good music however it's created, however it's motivated." - Thomas Newman