I honestly think it doesn't matter. I have about 10 years of professional experience, and while I used to write thank-you e-mails and even snail mail letters on every job interview I went on when I was younger, I stopped doing it a long time ago. Here's why:
a) A startup or a mid-sized company will usually act pretty fast in making an offer or informing the candidate they're declined. The bureaucracy at bigger companies may mean it could be awhile before you hear back from them even if they liked you, but for the most part I felt silly sending thank-you letters after my employment at the company was already determined.
b) It's common to go on an interview loop and meet 4-8 people. I found myself basically copy-pasting them same letter to everyone I had met and I felt really foolish. The alternative was to think of something unique to everyone, but this felt equally contrived ("I really enjoyed those questions about how Perl handles multiple inheritance!")
c) Business cards are pretty passe now and a lot of engineers don't get them at companies because they don't need them. So I would ask the employees I would interview for their cards, they wouldn't have any, so I had to awkwardly quickly jot down their full name and e-mail address while the next interviewer was already walking into the room. Or sometimes I would forget to ask, and then I'd e-mail the HR contact, and I doubt it ever got to the interviewer.
d) When I got older, I quickly realized I never made any difference to me as a hiring manager. If I liked a candidate, I hired him. If I didn't, I didn't hire him. If I received a thank-you e-mail, it didn't make a lick of a difference. I know it can demonstrate good communication skills, etc, but I already made my judgment on the candidate's communication skills in the 1 hour I spoke to him, not in the 15 seconds I took to read his thank-you e-mail.
e) At a company I interviewed at, I once sent everyone a thank-you e-mail, and then I was hired. Somehow my personal e-mail address was made the default in everyone's Outlook address book for my name, and it was literally weeks until it was fixed. Until then I had to keep checking my personal e-mail as well as my work e-mail. This is obviously more a failure of Outlook or that company's IT department, but that was basically the last time I ever sent thank-you e-mails.
Look, it won't hurt. If you're paranoid and getting a lot of 'old-school' advice, then go ahead and do it. But seriously, at no point does a candidate's offer status hang in the balance on his thank-you note. An interview debrief never ends with, "well, sounds like we could really go either way on this guy... let's see if he's the kind of polite young chap that sends a thank-you e-mail."
So good luck with the interviewing, and if you're concerned about coming off as polite and well-mannered, the best way to to express is it in the interview itself... not a contrived thank-you e-mail that the interviewer probably won't get anyway and won't influence his decision even if it does.