When one is fasting and not ingesting fats bile production is diminished and this creates conditions for the formation of liver/gallbladder stones more specifically on the longer fasts. It's always good idea to do a liver flush after. You want to wait till you've weaned off the MC properly and have started consuming some healthy oils so your system is producing bile again at a normal rate so your flush will be more productive. Be aware the the longer you fast the longer you should extend your breaking period.
While the MC is a great over all cleanse its not like you do these other cleanses to "get any last toxins". There is no cure all do all protocols including the MC and things like the liver and colon need to be addressed with dedicated protocols that the MC does not address fully. Like I always mention many times there are no compartamentalized issues and nothing works independent of everything else so if you want to be realistic about detoxing then you need to investigate all the other cleanses.
The liver flushes are not something you do just once it will generally take no less that at least half a dozen before noticing any results and in the neighborhood of a dozen or more before clearing the liver /gallbladder of stones. Liver flushes need to done 3 to 4 weeks apart. It took me 1.5 years and 18 flushes before I got the all clear of no more stones and I'm a realtively heathy person with no history of liver problems.