I find I am now taking note of these phrases and exchanges, and have decided that 'you're welcome' has quite serious import: I would only use it if something significant was being thanked for.

Mostly, exchanges consist of repeated thank yous, as mentioned above; at work in the library, I say 'thank you' when books are handed over (reply 'thank you') then 'thank you' when library card is handed over, 'thank you' when I hand both back, (reply 'thank you') to which I usually reply 'Thanks'.

Now, if somebody asked me to find a book for them for which I had to leave my desk, for example, and they thanked me for what I had done, I would say 'you're welcome' or even 'you're very welcome'!