Someone told me that you should never put a book of the scriptures on the floor or under your arm, but that it should always be placed in a high place. Is this true?
 
Until recently in Buddhist countries as in medieval Europe, books were rare and valuable objects. Therefore the scriptures were always treated with great respect and some of the customs you have mentioned are examples of this. However, while customs and traditional practices are alright, most people today would agree that the best way to respect the Buddhist scriptures would be to practice the teachings they contain.