Just because you add salt does not mean you won't have to shock ever again. Shocking a pool simply brings the FC level (Free Chlorine) up high enough so it eats away the CC (Combined Chlorine). If you "stay ahead of your water" as CarlD says, you'll rarely have to shock no matter what type of chlorinating method you choose. Adding salt will also NOT prevent you from adding more chlorine. As Mark has noted, you would need to invest in a Salt Water Chlorinator for that. Hopefully you have misunderstood and someone is not claiming such nonsense. If you are going to add salt, use solar salt or water softener salt. Check out these threads:
http://www.poolforum.com/pf2/showthread.php?t=683
http://www.poolforum.com/pf2/showthread.php?t=560
Dave