[development] Forms API #id questions

I'm in the habit of giving nearly every form element an #id because I've been doing lots of AJAX stuff and always have the need to grab things in the DOM. Can someone comment on this bit of instruction: http://drupaldocs.org/api/head/file/contributions/docs/developer/topics/form... Is the use of #id discouraged? Why is there no #id property allowed on hidden form elements? Thanks Robert