I know people will clone a clone and have done so for years with no reported loss in vigor or potency. I've heard two analogies describing this practice; cloning gives you the same exact genetic makeup as the mother, AND I've also heard that cloning a clone is like making a copy of a copy. Eventually you'll start to see some degradation in quality (or some genetic drift based on environmental stress).
Now, cloning a clone is a better practice than cloning a plant who has gone through flower, then revegged, but as far as I know, the most ideal situation is to have a mother in a perpetual state of veg who can supply clones at all times. This is how the professionals do it in Amsterdam as I've heard of mothers living up to 20 years. If the 'pros' do it a certain way, then it's probably a good idea to emulate them.
Maybe I'll run an experiment over then next few years as I keep several moms around which would act as a control. I could make sure that at least one plant is always the product of a cloned-clone and i'll do this to compare the results to the control. Hopefully it doesn't take 20 generations.