My apologies to Wynne for not explaining this concept better since he apparently didn't understand what I was speaking about and others may not have either.
The issue with adding all your minerals (calculated for the total batch volume) to the mash has little to do with the color of the beer or acidity of the grist. It has much more to do with the Residual Alkalinity condition you are developing with the calcium and magnesium additions. Its interesting that Wynne mentions pales, since that is probably the style that would be most affected by this issue since many brewers use a substantial gypsum addition in the brewing water to provide the beer with a high sulfate content. Since extra calcium will be added to the mash via that gypsum addition, that will have the effect of depressing the RA of the mash water. That reduced RA has the effect of depressing the mash pH. Since we are talking about using RO water as the starting point for the brewing water in this thread, the alkalinity of that water is near zero already. Adding a large dose of calcium to that low alkalinity water can really push the mash pH down. If the grist also includes a quantity of more acidic grains beyond the base malt, then it is entirely possible to push the mash pH below 5.2 (@ room temp).
I fully agree with Wynne that it is better to err on the low side of the pH range when brewing than to be on the high side. The beer will be more palatable in most cases. But every brewer should be aware that low pH also has negative effects in beer. Unfortunately, I've experienced this result first hand. Reduced hop expression, increased wort fermentability, and a notably tart flavor are examples of those negative effects. So, a brewer should pay attention and get their mash and wort pH in the right range instead of just assuming that everything will just work out. It is still best to mash in the 'right' pH range.