Personally I think if you have one item, putting it in the overhead is not a crime. If your coat is stuffed over it, no biggie there either.

For myself, I always put my laptop bag/backpack under the seat if there's room, but it's because I want easy access to my stuff. The trick of putting it under your knees after takeoff is something I employ often.

The person who has 2-3 items + coat and takes up half or a whole bin with all of them is the one I think is the actual target of this thread. One item up, and if you have more than one, one up, one under (except bulkhead of course).