The non-technical reason why there is loss when you save in jpeg format is because a higher priority is given to keeping the file size low. This so high quality images could be put on the web that loaded up quickly. It was always meant to be a 'one-off' operation - It is not a storage format IMO.
No problem with the originals ['negatives'] in jpeg, but if you need to make changes, I'd then store them in a different format that is lossless, as Risto and others have said