David,
This has been discussed many times and at length on the forum. The short answer is that everytime you save as JPEG there is some quality loss. The ammount varies by compression levels and if you resave after editing. Whether it is discernable to you is another question.
If you have a 10D, my preference would be to shoot in RAW and archive the unedited RAW images. Then if I save copies after editing it doesn’t really matter what format I choose, because I can always go back to the Raw source file.
If space is a problem, look into investing in a second hard drive. They are cheap these days and big.
Rich
Another solution is to get a CD burner (if you dont already have one). The major electronics stores have them on sale almost weekly for 9-25 dollars after rebate and you can usually get 50 blank disks for $12 or less (often a lot less after rebate)
Jpeg is a destructive format that should only be used for final output. If you value the quality of the originals, youll want to preserve them as is. Many cameras save as a jpeg doing the conversion automatically in the camera. If you are happy with the quality as it looks coming into the computer that is no big deal and you can keep those as is to be burnt on disk but if you open them to edit you dont want to save them as a jpeg again unless you are done with them and in such cases it is generally a good idea to save them as a copy.