Salt does have its place in ponding.  Salt acts as a  nitrite binder, mild antiseptic and will kill some parasites.  Always make sure you use an uniodized salt with NO additives, such as yellow prussiate of soda (YPS).  It's not recommended keeping salt in the pond year round.  

Salt should not be added in increments greater than 0.10% every 12 hours.  Two and one half pounds of salt will raise the salinity of 100 gallons of water by an increment of 0.30 %. One pound of salt will raise the salinity 0.12% in 100 gallons of water. 

 You can (and should) calculate the gallonage of your pond with the use of salt and a salt test kit.  To use the calculator that performs this task, you have to register (no charge) at this website: .  There's a wealth of koi related information here. Roark has a knack for making the complex seem simple.  

When dosed as a nitrite binder, salt should be dosed at a level of 0.10%.

When dosed as a mild antiseptic, salt will help with the healing of minor wounds in warmer water.  Salt should be brought to a level of 0.30% for this purpose.

Salt also kills some common microscopic parasites such as chilodonella, costia and trichodina.  Salt level should be brought up to 0.30% for 2-3 weeks.  There are some salt resistant strains that require higher levels of up to 0.60% or 0.90%.  At this level, your koi would probably be better off if you opted for another treatment.

Some pond plants don't fare well with salt.  It's something you should be aware of when dosing a pond with salt.  The main reason you don't want to maintain salt in the pond year round is because of the possibility of developing salt resistant varieties of parasites.