It is absolutely valid to use pre-created spreads, and there is no reason not to, especially if you are a beginner when it comes to reading the tarot. They are great jumping-off points. However, in my experience, when you develop your own personalized spread, the results are more in tune with your own nature and method of reading the cards. There is more of an intuitive flow. Also, it is very fun and satisfying to create a spread and see just how much more dynamically Spirit speaks to you through the cards. The magic of the tarot becomes very apparent when you develop your own spread and relationship to the cards, as opposed to rote memorization of spreads and card meanings.