Everyone's different on this one! lol! Personally, I keep my layered files for a couple reasons: in case I need to fix something (like a typo), if I need to adjust the margins to print in a book with different specs, or if I need to put together an album in crunch. Keeping ym layered files has been nice because if I need to put something together for someone else in a hurry, I can just switch out the photo and maybe tweak the title or journaling with minimal work.
I also save as a .tif instead of .psd because they can be around half the size sometimes... without a loss of quality! This is an option for Photoshop/PSE users. Not all programs are able to use .tifs. (They are still layered files just like .psd's)
Once I have a lot of layered files, I will burn them to dvd and maybe save them to my EHD and delete from my laptop's HD to save some room, but I want them somewhere.
I personally don't even make a full size jpeg until I'm uploading to print them somewhere... which is usually a book and not individual pages. I don't really care to keep my full size jpegs on my HD for very long after printing since I can quickly open up my .tif in Irfanview and make another full size jpeg if I need to. But I do need to burn the jpegs to DVD and maybe put them on my EHD.