Firstly to degrade a Jpeg you would have to save it thousands of time before you notices the image been degraded. Secondly, every time you work on an image use the save as option, to save the image as DSC_XXXX_ALTERED.JPG. In this way the original JPEG will remain unaltered.
I have a D800 and saving the JPEG, which is about 20mb as a TIFF, would result in a TIFF of 105mb. Each time you made changes to it, the file size would increase.
My advise is don't worry about saving the JPEG, as long as you save it at full strength, it won't make much difference.
