You don't need to have a water profile in mind except in the broadest sense. You do need to get mash pH right for best beer flavor. Mash pH has little effect on beer pH as that is set mostly by the yeast you choose to use. They will put the pH where they want it unless you make it difficult for them: high wort pH (think if it as a voltage) with lots of alkalinity (think if it as current) behind it, for example). A good value of mash pH for stouts is the same as for any other beer 5.3 - 5.6 should work out OK but you can expect, based on your own experiments, to find a subrange that suits you best for the particular stout you are brewing.
The difficulty with stouts is that it is such a broad category from the light, refreshing, dry Irish style through to the sweet, syrupy, rich heavy tropical stouts and RIS's. The only ones I brew are the first type so I feel competent to talk about them. You can, with a typical recipe and typical water brew without need for any alkalinity. This assumes sane levels of roast barley or black malt - up to say 20%. If you want to go that high then you will need alkalinity to control pH drop. You can also control pH drop by witholding the black malts as suggested but then you would have still set the mash pH to a proper level with just the base, probably using acid to do so and then when you later add the black malts you get their acid too so your wort pH is probably below 5. This is below what is generally recommended as a minimum wort pH but as I have never worked in that realm I can't say what the negative consequences, if any, are likely to be. AFAIK the commercial brewers of stout don't manage mash pH that way and I guess my feeling is that if you manage it properly you shouldn't have to use that technique either.
If you wish to mash just the MO and the 80L with RO water to which you add (per 5 gallons) 2 grams of CaCl2 and a gram of gypsum you could expect a mash pH of about 5.52 (with alkalinity of 100 that would go up to 5.65 or so. If you added 10% black malt to the 0 alkalinity water then you might expect to get mash pH of 5.44. Thus you would not need to withhold the dark malt at all at this level even brewing with 0 alkalinity water. Now were you to double that black malt to 20% (0 alkalinity) you might expect mash pH of 5.37 (and one of those stouts that leaves you, if you are unfortunate to be assigned to them in a competition, with a mouth that tastes as if you have been munching charcoal briquettes at the end of the flight). Note that the two decimal places in pH are not by any means justified in absolute terms because of all the assumptions I have to make but are included so you can see the relative effects. Using 20% with alkalinity 100 water would give an expected mash pH of around 5.47
Now in calculating all that I accidentally forgot to put in the calcium (right around 50 mg/L at the level of the salts I hypothesized) which would drop all the pH's I calculated by approximately 0.06 pH. Even so, you would, in none of the cases except perhaps 20%, 0 alkalinity be terribly uncomfortable with the results using no additional alkalinity. OTOH this certainly does focus bright light on Adam's suggestion that the salts be held back until the boil if you are uncomfortable with a pH of 5.32 (and I think I might be).
Now change any of the assumptions: use different base malts or different black malts or use over 20% black malt/barley and all this changes but it shouldn't change by that much. The idea you should get is that stouts are difficult to predict precisely (unless they are just plain old Irish stout) so that the best approach is test mash with pH meter.
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