In a math talk, is it better to display the number of slides left?
Halmos advises that in preparing a talk you should have a block of material that you can omit if you are running short of time.  But if the slides show number of pages left, you cannot do that invisibly.  For example: in an hour lecture, if you are 5 minutes from the end of the hour, but your numbering shows you are only halfway through your slides, the audience will be getting restless.