Excellent discussion on this issue. Sorry for the delay in my reply since I continue to see misinformation tossed about in the Forum.
I note that thcipriani has pointed out some inconsistencies that should be addressed. He did correctly point out a flaw in the rule of thumb that I proposed between RA and SRM. He pointed out that if you design a really dark beer, the required RA goes through the roof. I was using the typical maximum SRM for beers of 40 that is described in the BJCP Style Guidelines. With that color limit, the maximum RA that a brewer should need to use is about 180 ppm. That is supported by all the water profile information from the world's historical brewing centers. For instance, the brewing centers with alkaline profiles have RA's as follow: Munich = 180, Dublin = 170, Edinburgh = 150. Limiting the maximum RA used for brewing to 200 ppm is sound.
Unfortunately, thcipriani goes on to say that the correlation between RA and SRM is tenuous at best. Unfortunately he is quite wrong with that statement. He did provide a couple of references from AJ and Kai that actually do provide a correlation between roasted malt acidity and their color. Malt acidity consumes alkalinity in the mash, pushing the pH down. He is correct that there is not a precise correlation between beer color and RA, but there is a loose correlation that is adequately described and accounted for by the rule of thumb where: RA = SRM x 4.5. That correlation (along with a maximum RA of 200 ppm) will go a long way to correct the inappropriate correlations that exist out on the Web.
thcipriani goes on to say that the vast majority of water needs no adjustment. That statement is quite incorrect. The historical beer styles that grew out of the world's brewing centers are cases in point. There is no way that a brewer in Dublin could EVER hope to brew a good pale beer with their water and conversely, there is no way that a brewer in Burton could ever brew a good dark beer with their water WITHOUT ADJUSTING THEIR RESPECTIVE WATERS. The same thing applies to homebrewers where ever they are. Their water is probably suited to a limited color range of beers and if they want to improve their beers that don't fall within that color range, they will have to adjust their water chemistry. The only path to brewing a wide range of beers is to understand and adjust your water.
Now there is a slight flaw in what I stated above. My statement for needing to adjust waters from Dublin or Burton to brew other color beers is overly simplified. This is because there are many waters that already have too much ionic content and they cannot be adequately altered by ADDING minerals or acids to make them suited for brewing some styles. Sometimes you need to forget about using your local water if it has too high an ion content and you will need to resort to using distilled or RO water. This will help you avoid the alka seltzer or other odd flavored brewing results.
Its also humorous that thcipriani goes on to state that if brewers are worried about their mash pH they should get a pH meter and then adjust their mineral or acid content. He is espousing exactly the same thing that I'm stating with chemistry adjustment excepting that he is expecting a brewer to figure out what to do while potentially destroying a few mashes in the process. I'm more of a knowledge-based guy and I think that most brewers can figure out this stuff with the help of properly written guidance. My pH meter is a handy tool to have for double checking. But I found long ago that once you have figured out your water's characteristics, the pH meter will rarely be needed.
I appreciate the reference regarding the chloride/sulfate balance. I did read the reference that AJ posted on Brewing Network. I also researched the author of the water section of the Handbook of Brewing that the balance was mentioned in. Although the author, David Taylor, is an eminent brewing chemist and he worked at a number of fine breweries, I don't see a body of work in the brewing literature that suggest that he or others have actually researched and proven this. In reading further in that Brewing Network thread, AJ also tends to dismiss the applicability of that chloride/sulfate relationship. He suggests that maybe it works for English beers with English hop varieties. I will reaffirm that the use of the chloride/sulfate ratio is certainly not appropriate when either the chloride or sulfate concentrations exceed 100 ppm.
Regarding ionic balance, Tom Schmidlin is correct. Water reports or water profiles may not be balanced, but the ionic balance in water is ALWAYS balanced. Any imbalance in a water report is due to rounding error, detection error, or the presence of other ions that were not evaluated in the lab testing. Minor errors (say 5%) between anion and cation milliequivalents are fairly typical for water reports. If you find more than that difference in the anion/cation balance, then you should be questioning the results.
Regarding published water profiles for various world brewing centers, there are a bunch of water profiles published in a variety of texts and on the internet that are GARBAGE. Even though it is printed in a book does not make it correct. I have performed extensive research into the historic water sources and the water quality of those sources in resolving the appropriate constituents for those brewing center water profiles. That information will be published in the future.