Builders' construction techniques vary, but most people start at the bottom and work up and out. "The process of sculpting with LEGO is usually a bottom-up approach," says Sawaya. "Meaning, I start at the
bottom and build my way to the top of the piece. This generally means that balancing issues have already been dealt with by the time I finish a model."

From the Howstuffworks article, "How LEGO Bricks Work."