That is true, except that it has nothing to do with converting to a common format. If you have a jpg, and open it in gimp, make some changes, and then save it again, your image quality will degrade. The only solution is to always store your images in a lossless format, and only consider jpgs to be the "end result." Much like a programmer would never delete the source code after compiling, don't delete the original RAW, or TIFF, or whatever after creating a jpg.