the more necessary colonic seems to me, to be the one after the liver flush... the liver releases alot of toxins, in addition to any stones that come out. i do the colonic at that point to prevent the re-uptake of that toxic stuff.
also i don't think i could take doing a colonic both before and afterwards, since i'm doing a Liver Flush every 2 - 3 weeks - that would make 4 colonics a month - too much for me! so i stuck with the one that seems more important. i know Andreas recommends one before and one afterwards, but I know that I personally am not up for both. after each Liver Flush i've had a very bitter taste, which the colonic has always gotten rid of.