Even more important is to go over every line and don't go on to the next line until you are sure you understand. Just like you can read without understanding you can type without understanding as well. Forcing yourself to understand will make you a much better programmer.
Well, in one sense, I agree. Make sure you know what each line does on its own. But sometimes a single line doesn't make sense, in a larger sense, without something that comes later. So you have to have some tolerance for not understanding each line before you move to the next. It could be put in terms of knowing the "what" vs letting the "why" be uncertain a while.
This is very much in-line with the way the Learn to Code The Hard Way books are written. There's even a call to action at the beginning of each book imploring the reader to avoid copy-paste:
