Stress is usually not the cause of low blood pressure, unless you are going into some sort of fatigue syndrome (asthenia). The opposite is more common, actually are most of the cases of "essential" hypertension (high blood pressure without any obvious cause) related to stress and/or an unhealthy lifestyle in general.
If your doctor thinks you lack salt (assuming you mean ordinary salt, not other electrolytes) the problem is easily fixable by eating more salt (but of course, in moderation, don't eat salt in high amounts). Or possibly drink sport drinks containing electrolytes. Also make sure to stay hydrated throughout the day.
Low blood pressure is really just an advantage as it is protective against cardiovascular disease. Still, too low can cause a lot of symptoms (tiredness, dizziness, nausea, tinnitus) and in some cases fainting. Which is slightly ironic is that high and low blood pressure often may cause the same symptoms...
Good luck!