The only reason to add the burton salts to the mash would be to increase the pH. If you have a lot (more than 1lb) of dark roasted malts, then you may need some of the salts, partictuarly since your water is soft. 

Or put simply, after adding the malts, test the mash with a pH meter - if it's under pH 5.2, then add increments of 5g of the burton salts until the pH is around 5.1-5.3pH. If you don't have a pH meter, then I would still add the salts, since they are likely needed to provide some buffering capacity in the water against any dark malts in the grist.