If you drive in salt, yes.
It will help but there are still so many hidden areas than is impossible to seal with paint or undercoating and the salt will always get inside and keep eating up your steel. There is really only one solution unless you want your body to only last a few years and that is to stay far away from salt.
Note that the road salt you encounter in Norway is far more agressive than the salts used in most other european (including Sweden) or american (US/Canada) countries. Norway use the cheapest and most agressive form of salt (Sodium Chloride) while other countries have laws to use more expensive types of salts mixed with anti-corrosive agents.
Hope I dont sound too grim (just hate salt (on the roads, that is))