Over a period of time the stems will become thin on some strains. That is the only thing that I know of that becomes an issue. What that issue is remains debatable. Some say you just string the heavy buds and it's fine. Some say otherwise. I say get you some dang seeds and start again when the stems get to be toothpick thin. It's all about maximizing yields. Vigor is important to that process because a vigorous plant will grow faster, meaning less time waiting to flower. I am still figuring it all out but that's what I have found so far. You can clone a hundred times I have been told and not have any issues.