My philosophy regarding acidification is based on the alkalinity of the sparging water. I feel that the alkalinity should be 25 ppm or less. In the case of RO or distilled water use, its unnecessary to acidify those waters since they already have low alkalinity.
For neutralizing water with modest alkalinity, lactic acid is fine. You won't have to add enough to notice a flavor impact. I know from experience, that 150 ppm alkalinity is still OK for lactic usage. From studies I've seen, the flavor threshold for lactate ion is on the order of 400 ppm in beer (Malting and Brewing Science). One mole of lactate ion will be delivered to the water for each mole of bicarbonate neutralized. 400 ppm bicarb = 328 ppm alkalinity. So this suggests that there might still be a little room for using lactic acid on higher alkalinity waters, but I wouldn't recommend it. I'd say that moving to phosphoric might be safer from a flavor perspective. Another consideration is that the 400 ppm taste threshold represents the 'average' taster. A super taster might pick it up at lower concentration.
PS: Beersk, it sounds like you still have the 100 ppm alkalinity in the Sparge Acidification page inputs. If you put the alkalinity of RO water in that entry, it will report a much lower acid addition. This was fixed in the supporter version of Bru'n Water.