I've done the Liver-Flush the last day of the MC. Also done it during the Master-Cleanse when doing a long one (cause I do multiples during a long fast). I personally find the Liver-Flush takes a lot more out of me than the Master-Cleanse so I like to do on a Friday night so I can spend Sat on the toilet and have Sunday to recuperate before going back to work. I also find I don't need as many Epsom Salts as recommended -- since we're cleaned out anyways, the higher dose just really wrung me out. Your results may vary :-)
p.s. I did try it once after I'd been eating and it was very uncomfortable -- that's why I'd do it while or at the end of a fast..