If you're like me, very often your image files end up doing double duty: you use a high resolution version for printing (or, in my case, for my books) and another for posting to a blog, website, emails, etc. A lot of times though I'll need the lower resolution versions before I need the high-res files (to post to this blog, for example) and so I do a quick edit in low res first. The problem is that sometimes I get carried away in the low res edit and end up going much farther with the editing than I had planned; I then end up with a perfect edit only to find that I've done all that work on a low res file. Now, of course, I have to go back and try to recreate that nice edit in a high-res version--what a waste of energy! It's so much better and more efficient to do all of your editing in the highest resolution that you'll need (300 dpi if you're planning on printing the images) and then simply dupe that file and create a low-res version. That way both of your files are exactly the same except for the resolution and you only have to edit the image one time.
I can't tell you the number of times (particularly when I first started working digitally) that a publisher found an image on my main site and asked for a high-res file for a book project and I discovered I had never created the higher-res version. If I had had the better quality file ready, submitting it to them would have been a simple emailing job, but instead I ended up having to re-edit to match the low-res version. The image here (that I did edit in high res) took me about an hour to edit because I made selective curves adjustments on several parts of the image and then did selective color adjustments. I'm planning to use the photo in a new book, so fortunately I thought ahead and did my edits at 300 dpi and then just duped it down to 72 dpi (Image>Image Size) for web use. Also, I only put my copyright notice in the low res, so I do that last.
Edit things in the right order and create your master file first and you'll save yourself tons of time and energy.