But if your adding synthetics then salt build up is going to be an issue, for organic nutes no.. But you just told em up there to start adding synthetics.. which are going to build up in the soil if not properly flushed out?
Or am i smoking too much?
Nope your right. Just a small amount of synthetic to get them some available nutes. If he continued to use synthetic then yeas a small amount of run off can help but if you feed adequately you may not even need that with synthetic but it is good practice none the less.
With organics you don't really get a benefit to getting runoff you are then washing the available nutrients out of the soil.
Organics nutrients are not available until broken down and a lot less likely to cause toxicity because if this. It can happen but not as easily as synthetic nutrient that are almost entirely available