Water is not buffered, so regardless of the water used, if you add some sulfite to it, pH will rise to somewhere between 8.0 and 10.0 depending on concentration and temperature. So the 'need' to adjust pH is there regardless of the nature of the water one begins with. Btw, this 'need' is IMO kind of debatable to begin with since gelatin emulsions soften at a higher pH, which actually helps washing. Unless prints are soaked for an extremely long time in the solution (for which no need exists to begin with), it's not really necessary to reduce the pH of a sulfite solution. Plain sulfite will work just fine.