I don’t think it makes much difference, unless you have a slow network where the movie plays faster than the data arrives and you want to play a longer portion before it stops on an empty buffer and refills. I have also noted that with on-line video that there is enough data available to start playing a movie once 5% is loaded so if you have a 30% buffer it will wait 6x longer than it needs to before starting play. Again doesn’t mean much on a fast connection or local drive but it does when streaming from a slow server. As a result I use 10%. I may be totally wrong on this and others may have more info but it works for me.
