A spread that you create reflects your own thoughts, dreams and desires. Say for example you feel that the Celtic Cross doesn't appeal to you, because you are a pagan and the Christian symbolism offends you. You change the spread to suit your own desires. Say that the more common spreads, such as the Horoscope or, again, the Celtic Cross, employ too many cards, or the meanings that they assign each card don't speak to you: then you change the spread to reflect your own desires. There is no Tarot Police waiting to pounce upon you for altering spreads, or even the meanings of the cards, to suit what your own soul says those things mean. You can alter spreads so that they look good on a table, or reflect some special meaning to you personally, or in response to who knows what impulse. I knew a girl once that spread cards out in a Smiley pattern, the smiley smiling or frowning depending on whether the girl was in a good mood or not. I thought it was silly, but I didn't tell her so:
de gustibus non est disputandum.
Bob
edited to add: the Thoth fans may tell you to follow Crowley's advice: "Do what thou wilt shall be the whole of the Law." It seems to fit in this situation. (Bob)