Traditionally, when hand piecing, seams were pressed to one side to make them stronger. Hand piecing is done with a single strand of thread and pressing seams open would place a lot of stress on a single strand. The practice was continued with machine piecing, but machine pieced seams are much stronger so quilters also have the option of pressing open.
I press open when there are a lot of matching seams involved, as it reduces bulks where seams meet; otherwise I press to the side as it is faster.
Jinny Beyer, who hand pieces, doesn't worry about how seams fall. When she is done piecing she irons her top from the front side and lets the seams go any way they want. (Note she does not press them open!)