Personally I wouldn't take anything tatty or non-fiction that's out of date.
I'm surprised at the Oxfam assistant being snippy about paperbacks - that is unless they were very old and tatty - I buy quite a few of mine in the local Ox. bookshop and frequently donate 'non keeper' PBs there too. I never buy hardback fiction, whether new or 2nd hand - too heavy to read in bed!
I used to work in a library, where we were often offered donations, but many of them had to be chucked for the above reasons, v tatty or out of date NF. Sometimes we'd put donations on the for sale shelf, if we already had enough copies, but we just didn't have room for things that weren't going to sell, and presumably charity shops are the same.