Does it make sense to salt water after it boils? On the one hand, salted water has a higher boiling point. On the other hand, it increases regardless of whether you salt at one point or another  
As @Sneftel wrote, the amount of salt used in cooking will not change the boiling point of water by any appreciable amount. However, if you are adding salt to water for flavor or other purposes, there is a good reason to add salt only once the water is boiling (or at least hot), especially if you are using a stainless-steel pot. Salt crystals left undissolved at the bottom of a pot of cold water can corrode the stainless steel of the pot and result in pitting and discoloration. If you add the salt only once the water is hot/boiling, it will dissolve right away and not sit at the bottom to cause corrosion. 