When dealing with extract brewing, adding flavor ions is OK. But, there are caveats that all brewers should heed.
The first issue is knowing what the water you're mixing with your extract contains. If you're using distilled or RO water, then the baseline is easier to establish. If you're using tap water, then its still important to have a water report so that you can know the baseline condition of the water. The most important ions of interest to extract brewers are the flavor ions; sodium, chloride, and sulfate. Knowing what the bicarbonate content or alkalinity is also important to the degree that excessive bicarbonate (aka alkalinity) will be detrimental to beer flavor. RO and distilled water do not have excessive bicarbonate. With the baseline information on the ions in your brewing water, you can populate any of the mineral addition calculators. Bru'n Water also contains a mineral addition calculator.
The second issue is to avoid overdosing the wort with excessive levels of any ion. The unfortunate result of using 'Burton Salts' is that the brewer is left with little idea of the proportion of the minerals in the salt mix. They are probably mostly gypsum, but you just don't know. Starting with individual minerals like gypsum, calcium chloride, epsom salt, etc. is the only way to adequately assess what is added to your brewing water. Of course if you are brewing a malt focused beer, Burton salts would not be a good water addition. Armed with the baseline profile of the starting water loaded into the mineral addition calculator, a brewer can figure out how much of each mineral they might use in their brew without overdosing the beer. Any recipe that says 'add 1 tsp of Burton water salts' is providing bad advice. That might have been fine with the recipe originator's water, but might be a disaster if your water already has high levels of any of the flavor ions.
