I generally drink tons of selzter. Especially after Yoga when I am just tired of plain flat water.
However, on my first Master-Cleanse during 9/05 for 17 days I went by the book, and took no seltzer. My friend who introduced me to the cleanse said no.
However, on my 2nd cleanse about 4 weeks ago (11 days), I did drink some plain selzter from time to time, for a change of pace. And I had a great great cleanse. Not nearly as many side effects as the first. So for me, the selzter on my 2nd cleanse was acceptable. Other people may disagree.