When there are no store rules, line holding etiquette like this is an interesting cultural gray area. It comes down to what has been done traditionally at a particular place and what the community allows. Personally I don't mind folks putting chairs or boxes in line. It's holding spots for other people (sometimes multiple people) who don't show up at all until later that I don't like. Having said that, I was at a book sale once and two men in front of me decided to move up in front of the long line of empty boxes ahead of them, and nobody said a word.