I have no idea where the info at the top of the page comes from. I doubt it very much that there's anything that can form in a 2 week period that will increase any possibility of harm coming to you.
I do know that the Master-Cleanse works by giving the digestive system a rest and if you dump all that oil and grapefruit juice into your system in the middle of the Master-Cleanse you will be effectively stopping or serious hampering the cleansing process. I did my first Liver-Flush at the end on my first MC(40) with some great eye opening results. I've done more than 18 in a 2.5 year period an finally the last one I passed no more solid stuff so I'm hoping I'm done and will go on maintenace from now on.
Some folks say it's best to do it on the last day since the plumbing will be all clear and stuff will pass quite easily. Others have brought up the theory that it's best to wait a week or 2 after to let the body build up some bile. Each theory has it's merrits so pick one and bon voyage..........;+D